Some challenges for smart grid.
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Electricity is the fundamental enabler of human development. It permits technological advancements that are reflected in constant growth, while expanding its usage as demands increase. In order to produce electricity, several forms of energy with relatively abundant resources have been harnessed, such as hydroelectric, fossil fuels, and nuclear. However, major economic and sustainability factors throughout history have driven the energy consumption balance toward the exploitation of renewable energies. In fact, as these alternative types of primary energy are available at a variable rate defined by uncontrolled weather, its integration to the electrical network must accomplish a high level of control complexity in order to maximize generation without compromising grid safety.
Traditionally, power systems have had functional topologies that have served convenient routes from bulky generation power plants to load consumption centers. Moreover, distribution systems have been mostly designed radially although there are also possibilities to transfer circuits to other feeders in case of unscheduled disconnections. Despite the fact that the electrical network was conceived to transport energy vertically from generators to load, it has faced a major challenge to cope with the advent of renewable energies: the bi-directionality of the energy flow [1]. This characteristic aims to provide renewables sources to be distributed across the network in different sizes and at different locations, at the cost of increasing the number of interconnections in the distribution system, introducing new devices, and redesigning existing implementation practices. This paradigm has been fundamental to envisage the concept of smart grid [2], not only because of the energy shift but also the added intelligence the system must have to control such distributed scenario [3, 4]. Additionally, smart grids can contribute to grid survival in the case of natural disasters and large power plant blackouts. Thus, sustainability and safety are concepts that must fit in the smart grid landscape.
Although the distributed energy across the power system has technical advantages so far, much has to be done in order to make it stable and comply with operational and quality standards. There have been several approaches to study reliability improvements [5, 6, 7, 8], stability performance [9], communication technologies [10], and several other organizational transformations [11]. As an illustration, under the operational requirements for power system protection, fault-tolerant systems must discriminate the type of failure event based not only on its own measurements but also on its proximity. Thus, integrated communications systems are of uttermost importance in this case [12]. On the other hand, power quality issues must be compensated because other types of phenomena rising from the utilization of new switching technologies based on power electronics will emerge. Hence, maintaining voltage, frequency, and signal cleanliness even during rare extremely low probability events will become a must in new electrical energy devices. If all these conditions are met, the network operator can ensure the stability of the more complex power system. The future smart grid is an intelligent grid with higher levels of reliability and efficiency [13]. Some of the challenges that the smart grid must manage are detailed in Table 1.
Challenges | Application |
---|---|
Safety | Wide area monitoring (e.g., fault location) |
Greener resources | Integration of dispersed renewable generation and bi-directional customer (utility relation) |
Power operation | Improved demand control |
Automated power system operation | |
Energy quality improvement |
Some challenges for smart grid.
The newly demanded performance and functionality mentioned above cannot be obtained with current low frequency power transformers in the grid. These devices transform transmission medium-voltage electrical energy to consumable low-voltage electrical energy at 50/60 Hz frequency. Although it has proven to be highly reliable since power electrification days, it is not designed to handle distributed energy DC production and bi-directional power flow and does not have the capability to handle more complex control other than connection, disconnection, or voltage magnitude control (e.g., tap changers). Nevertheless, the distribution grid had an impulse of intelligence with the deployment of reclosing devices, thus adding more components to the existing infrastructure. This feature has been improved in the last three decades in order to adapt to more stringent conditions. But the introduction of local generation, power electronic devices, higher power requirements, and energy storage proved the current grid to be unable to handle all the operational challenges. Therefore, a new highly controllable modular device is needed to comply with the added complexity [14] of the network while maintaining quality standards. The solid-state transformer (SST) has shown to be flexible enough to accommodate several complex functionalities at different voltage levels with the advantage to be lighter and more efficient than the conventional power transformer and its recloser counterpart.
In fact, the SST provides the following features: availability of low-voltage DC link, power factor correction, VAR compensation, active filtering, disturbance isolation, and smart protection. The DC link allows the direct injection of distributed renewable energy into the grid. On the other hand, its other features add improved compensation and stability [15] for active and reactive power flow within a single smart device.
The SST concept is not only promising for the smart grid but also for other engineering applications. There have been successful attempts to introduce SST for traction process such as railway transportation, remotely operated vehicles (e.g., submarine applications for deep water exploration), and ship propulsion. Hence, there are a full spectrum of possibilities in which SST has shown to be a feasible alternative, for that reason it is sometimes known as the future “energy router.”
In order to illustrate the aforementioned capabilities of the SST, this chapter provides an insight into the operation of an SST. Nonlinear loads are revisited as an important part of the SST demand. Then, a mathematical model of an SST is detailed, and its performance under typical power system conditions and disturbances are analyzed. Additionally, a communication feature is also described, such that SST could not only be remotely operated but also take coordinated decisions to optimize power system operation and performance. Therefore, in the next sections, the advantages of SST are studied, demonstrating its feasibility for sustainable smart grid applications.
In order to study the benefits of SST in the smart grid, a system model that could represent its electrical properties is needed. Therefore, in this section the mathematical model of nonlinear loads, SST, and some common power system disturbances are described. Later, these models will be used to analyze power system disturbances and highlight the advantages SST has on the network operation.
Concerning the definition of a nonlinear load, it is necessary to specify linearity. Linearity is a characteristic used to describe linear loads, and it corresponds to a property in which loads exclusively produce fundamental sinusoidal current if supplied by a sinusoidal voltage source at fundamental frequency [16]. In contrast, nonlinear loads provide distorted current waveforms, thus injecting harmonic components in the system [17]. Load harmonics higher than fundamental frequency are commonly represented with a resistance-inductance-capacitance (also known as RLC) circuit in parallel with a current source, as shown in Figure 1.
Equivalent circuit of a nonlinear load.
Nonlinear loads act as sources of harmonic currents whose frequencies are multiple of the fundamental frequency. Harmonics circulated from the load to the source and, depending on the topology of the network, harmonic current can spread to other loads. These distorted current components may cause voltage spikes and terrible damage to nearby equipment. Note these phenomena in Figure 2.
The effect of harmonics.
The fundamental current waveform as a function of time
For simplicity, it is common to represent a sinusoidal function in its phasor form, where it is written as a complex number with amplitude and phase. The phasor amplitude is obtained from the root mean square value of the fundamental sinusoid function amplitude, that is,
In the presence of harmonics, waves are distorted and become a function of the total number of harmonics
Notice that the left-hand side term of the sum is the fundamental frequency sinusoid, which has exactly the form as presented in (1), while the right-hand side term is the harmonic current
By expressing (4) in its phasor form, the result is as given in (5).
where
Another simple way to describe the harmonic influence over the fundamental frequency sinusoid is the distortion factor
Solving for
Later, by multiplying (6) and (7), the expression is:
By replacing (8) in (9) and solving for
where
With respect to the vector representation of the phasor given in (2), the domains
Then the apparent current magnitude is
By replacing (10) into (12) and solving for
The last formulation describes in functional way the mathematical model for a nonlinear load.
An electrical disturbance is characterized by the deviations that it produces to the nominal voltage, current, or frequency conditions. These fluctuations can result in failure or abnormal operation on the system. These perturbations can be noticed as wave deformations affecting magnitude or frequency mainly. This effect is of uttermost importance in electrical utilities since they face the task to provide high-quality energy by regulation, in addition to balance generation and demand with adequate levels of electromagnetic compatibility that allows proper operation of electrical equipment.
Some equipment with nonlinear components, such as power electronic converters, electric arc devices, and others, cause problems usually related to electromagnetic interference (EMI). These disturbances cause a loss of performance in most conventional loads and unnecessarily overload in transmission or distribution lines. However, one of the most significant problems in addition to the performance degradation is the deterioration on the quality of the voltage sine wave, superimposing periodic or transient disturbances. This phenomenon jeopardizes the appropriate operation of electronic, computer, and communication systems.
Given the aforementioned problems, there is a need to formulate a model that could handle analysis and simulation. Table 2 shows the mathematical model and representation of the electrical disturbances analyzed in this chapter.
Electrical waveform disturbance mathematical model.
The SST allows isolation between medium- and low-AC voltage sides as any conventional transformer. Additionally, it allows the isolation and clearance of faulty conditions from both sides, as well as anomalies encountered in the AC or DC sides. Its DC link is highly attractive for the integration of photovoltaic energy, storage systems with uninterrupted power supply devices, or even future local DC grids. In order to accomplish all these features, its topology has several stages of power electronic blocks depending on the functionalities required. Thus, the SST can be designed depending on the type of application [23]. As a key technology in the implementation of the smart grid, its topology will heavily depend on the end user consumption and the integration and coordination features required. Some of these requirements are shown in Table 3.
Requirements | Description | |
---|---|---|
Integration | Renewable energies | Integration of distributed generation on LVDC (e.g., photovoltaic panels) or LVAC (e.g., wind micro-turbines) |
Storage systems | Integration of energy storage system (e.g., battery systems) or devices with UPS functionality | |
Coordination | Power quality | Voltage magnitude (e.g., power factor correction) Reactive compensation (e.g., fast response to voltage disturbances due to reactive energy unbalances) Voltage unbalance (e.g., rapid response to sags, swells, and all the harmonics originated at the load or perceived at the device’s input) Other quality events (e.g., electromagnetic transients, frequency variations) |
Remote operation | Communication functionalities to be integrated to higher management systems (e.g., SCADA systems, energy management systems (EMS), outage management systems (OTS), wide area management systems (WAMS), and other early awareness systems with synchro-phasor capabilities) | |
Consumption | Power supply | Several voltage level requirements: HVAC to LVAC, HVAC to LVAC + LVDC, etc. End user consumption such as LVAC loads (linear and nonlinear loads) and LVDC loads |
Various functional requirements for the SST.
As the modular arrangement of the SST depends on the grid requirements, several topologies have been proposed in the literature. Generally, the energy can be processed in three main stages: rectification, the same level AC-AC or DC-DC conversion, and inversion. Some of the available solutions to these stages are shown in Table 4. To provide a wider classification system for the SST, the level of modularity can be determined with respect to power flow direction, connection to three-phase systems, and connection to the medium-voltage level [24].
Rectification | Same level DC-DC conversion | Inversion |
---|---|---|
Full-bridge rectifiers | Buck/boost/buck-boost converter | Full-bridge inverters |
Multilevel cascade rectifier | Cuk converter | |
Active front-end rectifiers | Bi-directional DC-DC dual-active-bridge converter |
Typical power electronics topology for the SST stages.
A typical configuration of a SST consists of [25]:
Input filter, responsible for limiting the ripple of the input current, is composed of an inductor for each phase.
Full-bridge multilevel rectifier converts AC to DC voltage.
High-voltage DC (HVDC) link capacitors are energy storage for control purposes.
Dual active bridge (DAB) reduces the voltage level of each of the high links.
Low-voltage DC (LVDC) link capacitor is the link between the DAB and the three-phase inverter, which allows the integration of DC sources and DC loads.
Three-phase inverter converts the low link voltage into an alternating three-phase voltage.
LC filter is responsible for delivering an alternating sinusoidal voltage without distortion to the output of the SST.
To understand in a better way the SST topology, Figure 3 is presented.
Topology of the SST.
Focusing on the control system, it is divided into three stages: (1) multilevel cascade H bridge converter (MCHBC), which facilitates the conversion from AC to DC; (2) dual-active-bridge (DABC) DC-DC converter that allows the regulation of the energy in the low-voltage link capacitor and indirectly the DC voltage; (3) a three-phase inverter in series with a low-pass filter (inverter low-pass filter, ILP), which takes DC input wave and transforms it into AC without any distortion in the waveform. Table 5 shows the description of the electrical circuit and mathematical model of each stage of the SST.
Description of each mathematical model SST stage.
The deduction of each formulation is presented in [26], where
Once the SST mathematical formulation is defined, the drivers for each stage are designed. These are shown in Table 6 [26].
Description of each controller SST stage.
In this section, the SST is tested under different conditions. The analysis starts by describing the features of the SST, which are given in Tables 7–9.
Input voltage | Output voltage | Power output | Voltage modulation index for each converter |
---|---|---|---|
13. 8 kV | 440 V | 800 kVA | 0.85 |
SST nominal values.
MCHBC | DABC | ILP |
---|---|---|
Inductance and capacitance of each stage of the SST.
Stage | Control transfer functions |
---|---|
MCHBC | |
DABC | |
ILP | |
Drivers of each stage of the SST.
The grid is disturbed with a sag in the SST input. The sag appears with a voltage reduction of
Figure 4 reveals that even though the network voltage decreases (consequently the current injected also decreases), both the current and voltage on the load side are not affected. It is also observed that during the time the sag lasts, the inrush current increases. This increment is due to the SST control that keeps constant the output power, as shown in Figure 5.
Sag distortion waveform behavior: (a) grid voltage, (b) load voltage, (c) grid current, and (d) load current.
Sag distortion power behavior at the SST (a) input and (b) output.
The sag produces a decrement in the high DC voltage. To regulate it, the voltage modulation index (control) decreases. Its behavior is shown in Figure 6.
Control response to sag disturbance: (a) HVDC and its reference and (b) modulation index.
In this scenario the grid is disturbed with a swell in the SST input. The swell appears with a voltage increment of
Figure 7 shows that although the swell disturbance at the SST input, the voltages and currents in the load side are not affected. It is also observed that during the time the swell lasts, the input current decreases. This is attributed to the control of the SST, which keeps constant the output power; this fact can be appreciated in Figure 8.
Swell distortion waveform behavior: (a) grid voltage, (b) load voltage, (c) grid current, and (d) load current.
Swell distortion power behavior at the SST (a) input and (b) output.
The sag produces an increment in the high DC voltage. To regulate it, the voltage modulation index (control) increases. Its behavior is shown in Figure 9.
Control response to swell disturbance: (a) HVDC and its reference and (b) modulation index.
For this scenario, a nonlinear load of
Harmonic waveform behavior: (a) load current and (b) grid current.
Harmonic power behavior at the SST (a) input and (b) output.
In this simulation, an R-L load of 0.7 power factor is connected. Initially, the load operates with a value of 500 kVA; then at
Under these conditions, it must be verified that the power factor at the input is approximately 1 and that the output voltage maintains its nominal value. Figure 12 presents an increment in the magnitude voltage and a decrement in the current load; consequently the active and reactive power behavior is as given in Figure 13. The power factor in the load side does not affect the power factor at the input side (grid), as shown in Figure 14. It is verified that the SST can operate normally with an overload of 125%, and the power factor improves.
Overload waveform behavior: (a) grid voltage, (b) load voltage, (c) grid current, and (d) load current.
Overload power behavior at the (a) grid side and (b) load side.
Power factor behavior at the (a) grid side and (b) load side.
In this scenario, a distributed generation and an energy storage are connected to the DC link of the SST, with a voltage operation of 1144 V as presented in Figure 15. Initially, a load of 50% of their nominal demand is connected; later the load increases to 100% with a power factor of 0.85 lagging, as shown in Figure 16(c).
Electrical diagram for a bi-directional power flow.
(a) Generator active and reactive power, (b) generator power factor, (c) R-L load power factor, (d) power factor of the distributed energy source, and (e) power of the distributed energy source and the storage energy.
It is observed that at
It is possible to deploy a communication system that could satisfy the communication requirements and provide an enhanced operational capability in an SST network. Several types of topologies can be considered depending on the application. For instance, in a star-type topology, the communication linkage is established between each SSTs and the control center directly. Other topologies allow improved connectivity with alternate connections and meshed links. However, in all cases, a certain level of security, scalability, and minor delay in the information and bi-directional data transfer capabilities is required. While information capability performs digital monitoring of SST variables (as in SCADA systems), the bi-directional data transfer capability allows fast responses to disturbances such that system’s performance can be improved accordingly [27]. In fact, the smart grid (SG) concept is based on reliable real-time data availability and utilization for more intelligent decision-making.
There could be two forms of communication in SST networks: wired or wireless. Their selection depends on the bandwidth and the cost of the telecommunications infrastructure [28]. In the wired case, there are technologies based on power line communications (PLC) and optical communications and digital subscriber line (DSL) [29]. Table 10 shows the comparison of wired communication technologies for smart grids according to coverage range and maximum theoretical data transmission. It is observed that optical fiber main application is the connectivity between transmission/distribution substations, thus, forming large coverage areas satisfying very high volumes of data and low latency. However, the main disadvantage is its high installation and equipment costs. On the other hand, PLC and DSL are technologies that can be merged on existing copper-wired networks, but their bigger limitations are scalability and network flexibility [30].
Technology | Data range | Range | Use in smart grid | |
---|---|---|---|---|
Power line communication (PLC) | Narrowband PLC (NB-PLC) | NB-PLC: 1–10 Kbps (low data rate PHYs) 10–500 Kbps (high data rate PHYs) | NB-PLC: ∼150 km or more | NB-PLC: Large-scale automatic metering infrastructure (AMI) NAN/FAN WAN |
Broadband PLC (BB-PLC) | BB-PLC: 1–10 Mbps (long range) ∼200 Mbps (short range) | BB-PLC: ∼1.5 km | BB-PLC: Small-scale AMI HAN | |
Optical communications | Active optical networks (AON) | AON: 100 Mbps (up/down) | AON: ∼10 km | WAN NAN/FAN AMI (with FTTH systems) |
Passive optical networks (PON): BPON, -EPON GPON | PON BPON 155–622 Mbps (up/down) GPON: 155–2448 Mbps (up) 1.244–2.448 Gbps (down). EPON: 1 Gbps up/(down) | BPON, GPON: ∼20–60 km EPON: ∼10–20 km | ||
Digital subscriber line (DSL) | ADSL | ADSL: 8 Mbps (down) and 1.3 Mbps (up) ADSL2: 12 Mbps (down) and up to 3.5 Mbps (up). ADSL2+: 24 Mbps (down) and up to 3.3 Mbps (up) | ADSL: ∼4 km ADSL2: ∼7 km ADSL2+: ∼7 km | AMI NAN/FAN |
VDSL | VDSL: 52–85 Mbps (down and 16–85 Mbps (up) VDSL2: up to 200 Mbps (down/up) | VDSL: ∼1.2 km VDSL2: ∼300 m (maximum rate)–1 km (50 Mbps) |
Wired technologies for SG.
In the case where the installation is above ground level, SSTs could have a wireless communication system. In fact, whenever possible, wireless technologies are preferred due to its flexibility and low cost; they can cover difficult access areas (distant or inaccessible) in power system monitoring applications [31]. As an example, a multipoint to point (MP2P) communication system for SST-based power system is shown in Figure 17. There are several wireless technologies that depend on the coverage and data rate, and these technologies allow the adoption of the multilayer architecture for smart grid as shown in Table 10. In the case of an SST-control center communication network, it is also possible to incorporate different intelligent electronic devices (IEDs), remote terminal units (RTU), substation automation solutions (SAS), universal gateways, smart meters, etc. There will be an increased complexity in the network operation due to the large amounts of data. Hence, these types of applications will require higher reliabilities and lower latencies.
Communication network for SST.
There are several wireless technologies that depend on the coverage and data rate, and these technologies allow the adoption of the multilayer architecture for smart grid as shown in Table 10. In the case of an SST-control center communication network, it is also possible to incorporate different intelligent electronic devices (IEDs), remote terminal units (RTU), substation automation solutions (SAS), universal gateways, smart meters, etc. There will be an increased complexity in the network operation due to the large amounts of data. Hence, these types of applications will require higher reliabilities and lower latencies. For such complex networks, a geographical-dependent structure is required.
According to the geographical service, networks are classified in home area network (HAN), neighborhood area networks (NANs), and wide area network (WAN). These networks have different coverage areas as detailed in Figure 18. HAN refers to networks within a single point facility (e.g., substation); it can range from a single home to a business area network (BAN) or industrial area network (IAN). Outside the single point facility, there are NANs and WANs. NAN, also known as field area network (FAN), connects several HANS and covers the transmission or distribution areas within several square kilometers. On the other hand, WAN connects several NANs, and it is considered the backbone of the communication system. It can cover thousands of square kilometers including the main control center. WAN can be a hybrid network with a mixture of wired and wireless sections [32].
Communication layer for SST.
For applications that could be deployed wirelessly, the reader can find an updated selection of available technologies including satellite and mobile communications in Table 11. The communication spectrum could present congestion in licensed and unlicensed bandwidths due to the increasing number of technologies sharing the same resource. Therefore, the network designer must consider more stringent security mechanisms. A more efficient spectrum can deliver increased data rates and provide enhanced interoperability between devices and systems, as shown in the system architectures of Table 12. The main features for an efficient communication can be established through several qualitative and quantitative requirements for the SST-based power system telecommunications infrastructure, as shown in Table 13. It is important to highlight that many of the technologies of Tables 10 and 11 are integral in today’s power system operation, such as the advanced metering infrastructure (AMI), energy management system (EMS), wide area management systems (WAMS), etc. For the case of the SST-based power system, the wired and wireless technologies could provide a systemic integration and seamless communication (Table 12).
Technology | Data rate | Range | Use in smart grid | |
---|---|---|---|---|
WPAN IEEE 802.15 | 256 Kbps | Between 10 and 75 m | Vehicle-to-grid (V2G) HAN: AMI | |
Wi-Fi | IEEE802.11e (QoS-enhancements) | IEEE 802.11e/s: ∼54 Mbps | IEEE 802.11e/s/n: ∼300 m (outdoors) | V2G HAN AMI |
IEEE802.11n (ultrahigh network throughput) | IEEE 802.11 n: ∼600 Mbps | |||
IEEE802.11 s (mesh networking) | IEEE 802.11af: ∼26.7 Mbps | |||
IEEE802.11p wireless access in vehicular environments (WAVE) | IEEE 802.11ah: ∼40 Mbps | IEEE 802.11p: ∼1 km IEEE 802.11ah: ∼1 km IEEE 802.11af: >1 km | ||
WiMAX | IEEE 802.16 (fixed and mobile broadband wireless access) | IEEE802.16: 128 Mbps down and 28 Mbps up | IEEE 802.16: 0–10 km | AMI NAN/FAN WAN |
IEEE 802.16j (multi-hop relay) | IEEE802.16 m: 100 Mbps for mobile users, 1 Gbps for fixed users | IEEE 802.16 m: 0–5 km (optimum) 5–30 km (acceptable) 30–100 (reduced performance) km | AMI NAN/FAN WAN | |
IEEE802.16 m (advanced air interface) | ||||
Cellular communications 3G | HSPA | 14.4 Mbps down and 5.75 Mbps up | HSPA+: 0–5 km | V2G HAN: AMI NAN WAN |
HSPA+ | 84 Mbps down and 22 Mbps up | |||
Cellular communications 4G | LTE | 326 Mbps down and 86 Mbps up | LTE-Advanced: 0–5 km (optimum) 5–30 km (acceptable) 30–100 km (reduced performance) | |
LTE-advanced | 1 Gbps down and 500 Mbps up | |||
Satellite | LEO | Iridium: 2.4–28 Kbps Inmarsat-B: 9.6 up to 128 Kbps BGAN: 384 up to 450 Kbps | Depend on number of satellites and their beams | WAN AMI |
Wireless technologies for SG.
Smart metering and grid applications | Customer applications | Application layer | ||||||
---|---|---|---|---|---|---|---|---|
Authentication, access control, integrity protection, encryption, privacy | Security layer | |||||||
Cellular, WiMAX, optical fiber | PLC, DSL, IEEE 802.22 | Wi-fi, ZigBee, Bluetooth | Communication layer | |||||
WAN | NAN/FAN | HAN/BAN/IAN | ||||||
PMUs | Cap bank | Reclosers | Switches | Sensors | Transformers | Meters | Storage | Power control layer |
Power transmission and generation | Power distribution | Customer premises | Power system layer |
System multilayer architecture of SG [30].
Quantitative requirements | Qualitative requirements |
---|---|
Latency Reliability Data rate | Scalability Interoperability Flexibility Security Regulatory issues |
Network requirements for SST over SG.
The voltage supply should ideally have a waveform without deformations. However, nonlinear loads produce voltage waveform distortion that affects the quality of the grid, leading to a low energy efficiency. It is not possible to mitigate their presence since they have become part of daily life. Nonetheless, the implementation of smart devices (such as the SST) can hold on its effects, becoming in a potential solution to this problem.
The presence of SST in a power system can improve the power quality of the grid. The SST allows to uncouple the side of the network from the side of the load; then if a disturbance occurs from one side, it does not affect the components connected in the other side of the SST. In addition, the SST allows to enhance the power factor, support overloads, and keep nominal voltage on the load side, even though the input voltage is affected by either a sag or a swell. Another advantage of the SST is their DC link, which allows the integration of distributed generation and energy storage. The power coming from the DC link can deliver power to the network, if required.
Concerning the communication, the SST faces a great challenge. The requirements for SST’s wireless communication network are complex because they seek lower latency, greater bandwidth, interoperability, and scalability. For this reason, it is relevant to focus on involving other types of wireless networks as an alternative solution.
This study was supported by Escuela Superior Politécnica del Litoral (ESPOL), the Electrical System Research Group GISE of the Faculty of Electrical Engineering and Computer Science FIEC (ESPOL), the scholarship program Walter Valdano Raffo II (ESPOL), and the Secretariat of Higher Education, Science, Technology and Innovation of the Republic of Ecuador (SENESCYT).
No potential conflict of interest is reported by the authors.
Carbon dots (CDs) are nanoparticles generated from organic/inorganic sources, was first discovered in 2004 when single-wall carbon nanotubes were electrophoretically purified [1]. CDs can be classified as carbon nanomaterials that are less than 10 nm in size, they are the latest class of fluorescent nanoparticles [2]. CDs have attracted the interest of researchers in diverse fields of science and technology such as; optoelectronics [3], environmental pollution and remediation [4], biosensor [5], bio-imaging and biomedical applications [6, 7].
CDs possess properties such as being dimensionless, durable, large surface area, enhanced porosity and stability, ease of being functionalized, fluorescence emission, biocompatibility and low toxicity [5, 8]. These properties of CDs can be applied to improve the environment and human health [4, 9, 10].
A toxic rival to the CDs is the popular semiconductor nanocrystals popularly known as quantum dots (QDs). The QDs are a type of semiconductor nanoparticles with diameter range from 1 to 10 nm [11]. More so, QDs normally are made from semiconducting materials, especially iron and cadmium, which are highly toxic and expensive to acquire [12]. Compared to QDs, CDs are considered best option with a high degree of biocompatibility, cost-effectiveness and non-toxic. It also serves as a suitable substitute to QDs in numerous areas of research such as bio-imaging, bio-sensing, pharmaceutical and fuel cells [13, 14].
Carbon dots (CDs) are suitable for the modification of electrode sensors. It combines fundamental aspects of biology, chemistry, and physical sciences, computer science and electrical engineering to meet various needs in a wide application field. Therefore, carbon as a sensor portrays several meanings, conditional upon what field the user subscribes [15, 16].
Over the years, various bulk materials and several processes and techniques have been developed and adopted by a wide range of researchers in the synthesis of CDs. These processes include the hydrothermal and microwave-assisted routes, heating, biogenic synthesis, thermal oxidation, ultra-sonification, subcritical water process (use of oil bath and salt bath), refluxing and chemical oxidation [17, 18, 19, 20, 21, 22, 23, 24, 25, 26].
Three important factors must be considered in synthesizing CDs which are control of size, uniformity of CDs in solvents, and mitigated aggregation [27]. Wang and Hu [28] confirmed that CDs carbonaceous aggregation tends to form during carbonization but this can be prevented when synthesized by methods such as electrochemical synthesis, hydrothermal, or by pyrolysis method.
The application of biological and agro-waste to synthesize CDs have been advocated in numerous research such as; cooking oil waste [29], egg-white and egg-yolk [30], orange juice [6] as well as eggshells [31]. Though it is advantageous to use waste biomaterials in the synthesis of CDs to avoid competition with essential food production [32], however, the downside of the application of biomass in the synthesis of CDs is lacking of essential purity and structural homogeneity to obtain homogenous fluorescent CDs for purposes of sensing minute concentrations of analytes [21, 33]. These had caused the application of clean materials to be used in the synthesis of homogeneous fluorescent CDs [2].
A competent carbon source for soluble CDs synthesis is needed to comply with the goals of green chemistry and not be in direct competition with essential food production and should be cheap to synthesize [34, 35, 36]. Research in the synthesis of CDs must consider low price of additives and less purification steps in case of using biomass as a precursor material.
Thus, the emphasis is necessary on the cost of producing typical CDs, not to be a replica of the currently observed situation with semiconductor QDs, with the high cost and potential environmental negative impact and yet to achieve its full potential in commercial applications [37, 38, 39].
Carbon dots (CDs) have emerged to be attractive materials due to their excellent photoluminescence (PL) properties and wide surface areas, which are needed for sensitive and selective sensing of analytes [40]. These qualities are owed to the characteristics of the carbon element at nano-dimension and five valence electrons to bind carbon atoms [31, 41]. The green and sustainable carbons dots refer to CDs that are synthesized from agro and biomaterials that can be readily available without depleting their sources [42].
CDs can be obtained from various source [3]. These sources include plants and animal origins such as bamboo leaves, woods, green algae, sugar cane, mangosteen, carica papaya, saffron, gringko, neem gum, prawn shells, orange, cucumber and pineapple [32, 43, 44, 45]. Further interesting applications of CDs have been reported in diverse sectors of the environment and health fields of science and technology [3, 32].
For instance, Pattanayak and Nayak, in 2013 [43] presented an eco-friendly synthesis of iron nanoparticles from various plants and spices extract. The synthesis of nanoparticles from plant parts (leaf) is essential since this will not require expensive processes that are involved mostly in biomaterial processing. Iravani et al., [46] demonstrated a green synthesis of metal nanoparticle using plants (emblica officinalis fruit extract) as a mean of mitigating the synthesis process of metal nanoparticles that are efficient and able to enhance green chemistry procedure for nanoparticles synthesis.
Liu [44], reported a research work on one-step green synthesized fluorescent carbon nano-dots from bamboo leaves for copper (ll) ion detection and demonstrated the exploration of bamboo leaves as a carbon source. Carbon nano-dots were synthesized hydrothermally and a resultant high quantum yield quantum dots, with sensitive Cu2+ detection at a limit of detection as low as 115 nM on a dynamic range from 0.333 to 66.6 μM. The zeta potential of the pristine carbon quantum dots was measured at −4.78 mV which changes to +13.8 mV after treatment with positively charged polyethyleneimine (a water-soluble cationic polymer).
Wembo et al., [47], researched on the economical and green synthesis of fluorescent carbon nanoparticles and their use as probes for sensitive and selective detection of mercury (II) ions. The adopted process by Wembo and colleagues was based upon the economy and green preparative strategy toward water-soluble fluorescent carbon nanoparticles with a quantum yield of 6.9% by a hydrothermal process using a low-cost waste from pomelo peel as a carbon source.
Piyushi, et al., [45] cultivated chlorella (a genus of single-cell green algae belonging to the phylum Chlorophyta) on brewery wastewater for nanoparticle biosynthesis. The method of bio-nanoparticle synthesis using chlorella algal biomass grown in single water sample were harvested from the culture medium by centrifugation at 4000 rpm for 5 min followed by washing with ultrapure water to eliminate impurities. Iron nanoparticles were synthesized by mixing 0.5 g (dry weight) Chlorella sp. MM3 with 5 mL of 0.1 M FeCl3 solution followed by incubation at 37 C for 48 h which entails long and tedious process.
Till et al., [48] synthesized CDs by microwave-assisted hydrothermal treatment of starch and Tris-acetate-EDTA. The process confirmed that nitrogen-doped CDs have emerged to be complementary to starch-derived CDs. Addition of nitrogen to CDs improved the yield of photoluminescence from 19% to 28%, making them promising luminescent materials for improving fluorescence of CDs. However, there is no added value in incurring additional chemicals during synthesis process of CDs. Starch is a better alternative to the use of nitrogen for synthesizing CDs. Till and colleagues observed the effect of nitrogen (N) additives, through the use of ethylenediaminetetraacetic acid (EDTA); tris (hydroxymethyl) aminomethane (Tris) and a combination of both (TAE-buffer) on the photophysical properties of CDs. Temperature (45 min at 230°C) plays an important role in the improved nitrogen-doped carbon structures [48].
Some researchers have adopted nitrogen for fluorescence and photoluminescence enhancement, but this approach has shown indistinct composition which required extensive purification steps. This, however, is environmentally not suitable and contravenes the concept of green chemistry since it involves many chemicals in the synthesis process [49].
Synthesis methods of CDs can be divided into two major parts; top-down and bottom-up as in Figure 1. Top-down starts from cutting the carbon materials into carbon particles or cleavage of larger carbonaceous materials such as carbon nanotube by laser ablation, arc discharge, electrochemical and candle/natural gas burner soot, and recently the hydrothermal route.
Synthesis methods of carbon dots (CDs).
The bottom-up route involves the use of molecules as support for localizing the growth of CDs by blocking aggregation during high-temperature treatment. However, this study explores the top-down process of CDs synthesis. As earlier stated the top-down approach concentrates on precursor carbonization that include microwave-assisted method, chemical oxidation, heating, and hydrothermal process [50].
It is a process removal of material from solid or liquid by irradiating it with a laser beam [51, 52, 53, 54]. Material evaporates or sublimates when the laser flux is low and converted to plasma at high laser flux. Goncalves and colleagues [51] reported the synthesis of CDs from carbon targets immersed in deionized water by direct laser ablation (UV pulsed laser irradiation). CDs were optimized and synthesized after being functionalized with NH2– polyethylene-glycol (PEG200) and N-acetyl-l-cysteine (NAC). To produce particles in tens of nanometer range by laser ablation, the energy is controlled within the incidence area of the precursor [54, 55].
Yu et al., [53] demonstrated the possibilities of relying on irradiating a toluene sample with a non-focused pulsed laser that is very different from the high powered laser irradiation employed in conventional ablation. This process by Yu and colleagues revealed an induced transformation of toluene into graphene sheaths, which subsequently produced fluorescent CDs. These nanoparticles can simply be functionalized using more than one molecule and stayed stable in an aqueous solution. It can also be applied to optical fiber devices through immobilization due to its stability in a specific optical nano-analytical sensor [56]. However, the equipment to conduct laser ablation is quite expensive and it needs technically skilled personnel to operate.
CDs were first discovered through this method accidentally when the separation of single-walled carbon nanotubes (SWNTs) were made using gel electrophoresis from carbon soot by arc discharge method. Carbon is formed when direct current arc voltage is applied in an inert gas across two graphite electrodes. The biggest challenge of this method is that it generates impurities that are difficult to purify [1].
Electrochemistry is another top-down approach used in synthesizing CDs, this process is facile and the product yield is normally high [57]. CDs with a size of 6 to 8 nm and 2.8% to 52% can be obtained through exfoliation that utilizes graphite rods and Pt wire in ionic liquid or water solution.
The mechanism of the exfoliation was due to complex interplay of anodic oxidative cleavage of water and anionic intercalation from the ionic liquid using titanium cathode and spectrum pure graphite in the center of electrolyzer to yield pure blue fluorescent CDs without the urgency of complex purification [28, 58, 59, 60, 61, 62].
Application of carbon soot in the synthesis of CDs have been reported by Tian et al., [63], the carbon source was from a carbon-processing reaction. Due to the simplicity to obtain the starting material, this method has been used widely by researchers. It also provides a new use for a complicated by-product. At the same time, it possesses disadvantages such as uncontrolled chemical surface, production of many byproducts that can harm human health with a broad dispersion [64].
Wang et al., [13] prepared CDs by microwave method. It proved reaction time can be shortened to 30–45 minutes with microwave-assisted technique. Similarly, Choi et al., [10] made effective use of lysine as a precursor to synthesize CDs within 5 minutes in a home type of microwave oven and the CDs were soluble in water with deep blue photoluminescence at a high mass yield of 23.3%.
Compared with other methods, the microwave route is more convenient since the heating of the carbon precursor is rapidly achieved within few minutes. It also exhibits high quantum yield and provides a long fluorescence lifetime. The procedure of microwave synthesis is much easier compared to others as it only utilizes heating via irradiation technique [65].
This method is mostly applied to produce CDs on an industrial scale. CDs can be obtained through oxidation treatment of carbon precursors by a strong oxidant. CDs from natural products have been researched and developed, by the synthesis of large scale CDs from human hair, coffee, and biomass by adding it into concentrated sulfuric acid and then heating at different temperatures. The time range is from hours to days [66]. By varying the temperature of synthesizing CDs, the quality of CDs such as diameter and quantum yields can be controlled [67, 68, 69].
The hydrothermal route of synthesizing CDs is considered as environmental-friendly, low cost and involves few synthesis steps that are non-toxic [2, 70, 71, 72, 73, 74]. Musa et al explored the hydrothermal method at a temperature range between 75°C to 175°C where the researchers reacted the precursor in a sealed hydrothermal reactor that resulted into a high yield photoluminescent quantum yield at 34.9% [2].
As illustrated in Figure 2, tapioca was added to an aldehyde solvent (acetone + sodium hydroxide) to improve the mobility of glucose molecules in starch [2]. The mixture underwent stages of reactions such as hydrolysis, adsorption, and gelatinization to particle disintegration simultaneously [2]. The carbonization temperature breaks the bond between the starch, making it available for the reactive solvent which leads to hydrolysis to form disaccharide and gelatinized glucose. The disaccharides polymerized into polysaccharides and the gelatinized glucose yielded CDs for functional group characterizations [9].
Mechanism for synthesis of carbon dots [2].
Like all-natural products, starch undergoes seasonal changes and particularly the amylose/amylopectin ratio is influenced by plant species and area of plant cultivation, which could influence the CDs formation. Independently of seasonal changes and origin, a starch will provide CDs with highly reproducible photoluminescent properties [2].
Substances such as glucose, citric acid, banana juice, and protein are examples of many precursors used to prepare CDs by adopting the hydrothermal route of synthesis [75]. Success has been reported in the synthesis of CDs through one-step hydrothermal carbonization using chitosan applied directly as a bioimaging agent [76]. The hydrothermal method is promising in producing CDs and is suitable for industrial or large scale production [75]. However, it is notable in 2010 where Zhang et al. [77] first reported a one-pot hydrothermal method to synthesize CDs from ascorbic acid in the presence of ethanol as solvent. Quantum yield and average particle sizes of their synthesized CDs were 6.79% and ~2 nm, respectively [77].
Several methods of synthesizing CDs has been explored in this section, to prevent the use of expensive precursor and energetic systems like in laser ablation. The hydrothermal synthesis route is being recommended as foremost for the sake of ecological sustainability [25]. Chemical oxidation and exfoliations provide an inexpensive alternative although it employs large amounts of strong acid which is hazardous and undesirable [77].
The other methods of synthesizing CDs need multi-step experimental operations and some of them require post-treatments to improve their water solubility, stability, and luminescent. Besides, several other methods suffer from drawbacks such as they require complex process and high temperature, time-consuming, harsh synthetic materials, and are expensive. This causes their applicability to be limited [78]. Several research successes proved that hydrothermal route to be a green method for the synthesis of CDs since the procedure produce soluble fluorescence CDs at reduce time and cost [2, 46, 79].
CDs derived from organic sources are excellent for the researcher and environment. Because the adoption of such material presents the choice to eliminate the need for metallic quantum dots, and any doping requirements, either through the use of sulfur (S) or nitrogen (N) agents. The use of metallic quantum dots and possible inclusion of S and N in enhancing their functionality contravenes the purpose of sustainable applications of nanomaterials in the modern field of nanotechnology [48].
Table 1 is a list of different synthesis techniques that have been attractive to researchers in recent years. The table provides a list of interesting techniques such as hydrothermal, microwave assisted, biogenic synthesis, thermal oxidation, ultrasonication, refluxing and chemical oxidation with excellent particle sizes [80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91]. The hydrothermal synthesis of CDs proves to be efficient and effective since it provides relatively smaller sizes of the nanoparticles as synthesized by Du et al., [81] at 1.8 nm, when compared to other methods such as chemical oxidation by Thambiraj and Shankaran [85] at 4.1 nm, biogenic synthesis by Phadke et al., [20] at 5–8 nm, and refluxing by Himaja et al., [84] at ~50 nm.
Method | Size (nm) | Reference |
---|---|---|
Microwave-assisted | 2.7 | [7] |
Microwave | 5–10 | [10] |
Biogenic synthesis | 5.0–8.0 | [20] |
Thermal oxidation | 5.0–10 | [22] |
Heating | 3.0 | [78] |
Hydrothermal | 2.3 | [80] |
Hydrothermal | 1.8 | [81] |
Ultrasonication | 5.0 | [82] |
“Oil bath” | 2.59 | [83] |
Refluxing | ~50 | [84] |
Chemical-oxidation | 4.1 | [85] |
Chemical oxidation | 2.5 | [86] |
Carbon dots sizes and synthesis techniques.
Note: n/a = not available.
One of the CDs properties is that it shows strong optical absorption in the UV region (200–800 nm) with a tail extending to the visible range, see Figure 3. CDs possess low toxicity with excellent photostability as compared to semi-conductor quantum dots [7, 23, 50, 71].
Optical properties of carbon dots at UV-visible absorption and emission spectra.
Absorption shoulders in the spectrum are due to the π-π* (pi to pi star transition) of C〓C bonds or n-π* (n to pi star transition) of C〓O and other fringe functional elements present [69, 92].
The uniqueness of CDs is the availability of wide surface area for trace detection of analytes and provision of adsorptive sites through the availability of heteroatomic carbon in nano-dimension along with photoluminescence emission. Based on past study, CDs is dependent on intensity and wavelength emission towards its excitation wavelength [93]. This is due to the different sizes of particles and surface chemistry and/or different emissive traps on CDs\' surface. The wavelength dependence behaviour makes CDs possible to be applied in multi-colour imaging and adsorptive purposes. Vinci et al., [93] suggest that CDs\' core, surface states, and size are responsible for their emission and adsorptive properties [93].
Table 2 shows the excitation wavelengths of CDs through the UV-lamp excitation process to obtain fluorescent characteristics [2].
Colour | Interval of wavelength (nm) |
---|---|
Red | 700–635 |
Orange | 635–590 |
Yellow | 590–560 |
Green | 560–520 |
Cyan | 520–490 |
Blue | 490–450 |
Violet | 450–400 |
The color range of visible light spectrum.
The colour of CDs most of the time is related to the surface groups which corresponds to particle sizes [93]. Normally CDs show strong photoluminescence from blue to green wavelength. To enhance the quantum yield (QY) of CDs or change photoluminescence (PL) emission to meet desired applications, surface passivation and functionality play a vital role. Besides, CDs show great photostability as there are no reductions in PL intensity with continuous exposure to excitation. In terms of chemical properties, different synthesis methods of CDs lead to different chemical structure and abundance of surface sites. They are usually connected or modified by polymer chains, oxygen-based, amino based groups, and others [93].
Characterization of CDs by high resolution transmission electron microscopy (HRTEM), Xray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), Atomic force microscopy (AFM) and Zeta Potential provide deep insights into the attributes of CDs such as hybridization and coefficient between functional groups and carbon core that take parts in the provision of the abundance of surface sites and the photoluminescence behaviour [94]. In comparison to graphene and metallic quantum dots, the CDs serves as the way out of toxicity concerns in environmental monitoring and medical applications [95].
The sizes and texture of CDs are important for fundamental applications in the field of environmental science and nanotechnology. Figure 4(A–C), shows the HRTEM images of CDs at different resolutions between 1 nm to 10 nm. Synthesized CDs revealed amorphous quasi-spherical morphology with a lattice spacing of ca 0.24 nm (Figure 4A), CDs characteristics are suitable absorbent of pollutants that are larger than 0.24 nm [2, 71, 89].
High resolution transmission electron microscopic (HRTEM) (A) Lattice space of carbon dots (CDs) characterized in magnifications of 5 nm. (B) Images of CDs at 10 nm. (C) Size distribution of CDs within 10 nm magnification [2].
High-Resolution Transmission Electron Microscopic images of the CDs characterized in magnifications of 5 nm and 10 nm (Figure 4A and B respectively). Figure 4A is the lattice spacing for carbon dots at 5 nm magnification. While Figure 4B is the size distribution within 10 nm magnification. Figure 4C is the histogram chart, demonstrating the nanoparticle sizes of CDs. The synthesis of nanoparticle with low lattice space is needed for research applications of CDs in environmental chemistry, pollutant entrapment in aqueous media and water purification [74]. The interplanar distance (lattice spacing) of 0.24 nm (Figure 4) is lower than the lattice spacing planes of graphitic materials (0.34 nm), the larger interlayer spacing could be attributed to the abundant oxygen-containing groups. In other words, the oxygen-containing groups could expand the layer spacing. The synthesized CDs is in consonance with recent reports by Arumugam and colleagues, CDs was hydrothermally synthesized from broccoli [79], ginkgo fruits [87], and cabbage [8].
Fourier-transform Infrared spectroscopy (FTIR) portrays the functional structure of CDs. It reveals the intrinsic functional groups and other useful compounds present in CDs. Figure 5 provides functional groups that exist before and after the hydrothermal treatment of tapioca as a precursor for CDs.
FT-IR spectrum of the carbon dots and tapioca [2].
As shown in Figure 5(A) representing tapioca. Peaks associated with the stretching vibrations of hydroxyl (▬OH) and carboxylic (COO▬) groups are at 3353.45 and 2933.78 cm−1 [75]. Further stretching vibration of C▬H occurred from 1645.24 to 1341.82. The peaks at 1151.38, 1079.20, 1014.41 cm−1 can be due to the C▬O stretching vibrations and out-of-plane bending modes of sp2 and sp3 ▬CH group [75].
There were substantial changes observed in the spectra of CDs (Figure 5B). The hydroxyl (▬OH) group of 3389.71 cm−1 increased on the carbon structure as a result of hydrolysis. While the carboxylic (COO▬) group 2145.73 cm−1 reduced by thermal destruction of saccharides structure [34]. The peaks at 1695.27 cm−1 and 1644.62 cm−1 showed the increase in the C▬H stretching vibrations of the bending modes of the sp2 and sp3 ▬CH group. The peaks around 1427.63 cm−1 until 1369.43 cm−1 are due to C▬O▬C [34]. The peak at 1237.62 cm−1 corresponds to the C〓C stretching vibration while 1094.19 cm−1 and 996.19 cm−1 represents the C〓O stretching vibration and the last group at 706.78 cm−1 denotes the C〓C bond of the unsaturated glucose structure in the starch. These attributes were responsible for the water-soluble nature of CDs [34]. The FTIR graph shows the formation of unsaturated carbon. Along with oxygen-rich groups such as hydroxyl, carboxyl, and carbonyl situated on the CDs surface, which agree with the hydrothermal synthesized CDs from the organic origin [23, 25, 26, 81, 90].
There are numerous applications of carbon nanoparticles due to the abundant properties they possess [3]. These applications are being discussed in the subsequent sections of the report.
Carbon dots have shown great potential to act as a sensor and can be used for environmental monitoring and control of pollutants, more so in the medical field for biosensor applications. It can donate or accept electrons that make it suitable for detection of ions, vitamins, nucleic acid, protein, enzyme and biological pH value [7, 11, 96, 97, 98]. Even though different materials are used to detect specific ions, the detection mechanisms are identical [99].
The functional groups on the surface of CDs specify distinctive affinities to different target ions, through an electron or energy transfer process and high selectivity to other ions [100]. CDs has been involved in the detection of 2,4,6-tri-trotoluene (TNT) and also applied as a dual-sensing platform for fluorescent and electrochemical detection of TNT [101]. Other reports utilized CDs as pH sensors for in-vitro and in-vivo investigations [102].
Research showed CDs able to detect intracellular pH inside a living pathogenic fungal cell and has been developed to sense nucleic acid in the DNA [103]. In other cases, CDs have been used in bioimaging because of their low toxicity and excellent photostability compared to semi-conductor quantum dots that posed health problems and environmental concerns [8]. Its visible excitation, emission wavelengths, and high brightness confirm CDs as a suitable candidate in this area. Several studies have been conducted using CDs in cell imaging, including pig kidney cell line [104], Escherichia coli [105], Hela Cells [106], liver diseases [95], see Figure 6.
Graphical description of fluorescence images of carbon dots.
Chengkun et al., [98] discovered photoluminescence in CDs synthesized from Nescafe original instant coffee and applied it in the field of bioimaging. From their investigation, CDs from Nescafe are found to be amorphous and the cytotoxicity study revealed that the CDs did not cause any toxicity to human hepatocellular carcinoma cells at a concentration as high as 20 mg/ml. Yang et al., [107] also worked on novel green synthesis of high-fluorescent CDs from honey for sensing and imaging. It was an innovative and green approach towards a CDs of high fluorescent quantum yield and excellent photostability, employed for HeLa cells imaging and coding. Rui-jun et al., [108] produced photoluminescent CDs from polyethylene glycol (PEG) for cellular imaging. The PEG is a biocompatible non-conjugated polymer, used as both carbon source and passivating agent [108].
The application of CDs in the selective detection of heavy metals have been reported in several scientific and experimental research [34, 40, 79]. However, there are gaps and lapses needing redress, such applications are predominantly in photoluminescent quenching of heavy metals. whereas, current section looks into reliable and robust CDs for applications in electrochemical sensing of multiple ranges of heavy metal ions.
The development of a convenient and sustainable technique for detecting and identifying human and environmentally toxic metal ions is of great interest. The following are reports concerning CDs application in heavy metal detection.
Zhang and Chen [109] worked on nitrogen-doped carbon quantum dots application as a turn-off fluorescent probe for the detection of Hg2+ ions at a detection limit of 0.23 μM. The fluorescent quenching mechanism is attributed to the surface-state triggered by the mercury-induced conversion of special functional group (▬CONH▬) from spirolactam structure to an opened-ring amide [109].
Sandhya et al., [110] applied nanostructures for heavy metal ion sensing in water using surface plasmon resonance of metallic nanostructures. They reviewed on techniques to improve selectivity and sensitivity of surface plasmon response sensors with attention to homogeneity. Effects of particle size, shape, material type, and surrounding environment were found to be effectual in the surface plasmon surface frequency.
Similarly, Qu et al., [111] developed CDs to detect Fe3+ ions by using dopamine as a starting material with a detection limit of 0.32 μM. Quenching of photoluminescence intensity occurred when there was an interaction between CDs and ions. Meanwhile, Liu [44] reported a research work on one-step green synthesized fluorescent carbon nanodots from bamboo leaves for copper (ll) ion detection and demonstrated the exploration of bamboo leaves as a carbon source with high carbon constituent. Carbon quantum dots were synthesized hydrothermally with sensitive Cu2+ detection at limit of detection as low as 115 nM and a dynamic range from 0.333 to 66.6 μM. The zeta potential of the pristine carbon quantum dots was measured at −4.78 mV which improved to +13.8 mV after treatment with positively charged polyethyleneimine (a water-soluble cationic polymer). More so, Rao, et al., [112] reported on the ability of CDs generated from citrus acid anhydrous to detect heavy metal such as Fe3+, with a detection limit of 0.239 μM.
Methionine has been used as a material for the synthesis of CDs [113]. These CDs were co-doped with nitrogen and sulfur to enhance surface functionalization for the detection and environmental monitoring of heavy metal pollutants [113]. Similarly, Shen et al., [4] applied fresh pomelo in the synthesis of CDs co-doped with nitrogen and sulfur for the detection of chromium (Cr (VI)).
A fluorescent probe for selective detection of metal ions such as mercury (Hg2+, 1.00 × 10−8 − 1.50 × 10−3 M, 1.00 × 10−7 M) with wide linear range and satisfactory detection limits was discovered when citric acid monohydrate was used for the synthesis of fluorescent CDs [114]. More essentially and effective is the burning of ash from waste paper and further utilized as a source of CDs by Lin et al., [115]. They succeeded in synthesizing CDs without any surface modification and subsequently, the fluorescent CDs were quenched by Fe3+.
Simpson et al., [21] synthesized carbon nanoparticle from glycerol and phosphoric acid mixed in a Berghof high-pressure reactor at 250°C for 4 hours. Afterward, glassy carbon electrodes were fabricated by drop-casting the carbon nanoparticles, and further applied for heavy metal (Cu2+ and Pb2+) detection by square wave anodic stripping voltammetry [21]. Heavy metals such as Na+, K+, Mg2+, Ca2+, Cr3+, Co2+, Ag+, Hg2+, Cd2+, Pb2+, Ni2+, Cu2+, Zn2+, Al3+, Fe2+, and Fe3+ have been tested on CDs synthesized from carbon source of mangosteen pulp and a ground discovery was made. Among the listed heavy metals, Fe3+ was the favourite in detection with a detection limit of 52 nM. Further application was found for cell imaging, which reveals their diverse potential applications [89].
Abhishek et al., [14] made a paper strip based live cell ultrasensitive lead sensor using CDs synthesized from biological media. They reported a formulation of a sensor through microwave heating of potato-dextrose agar (PDA) for the detection of lead (pb2+) in solution but again involved a long and laborious process.
Pajewska et al. [116] explored the fluorescence of synthesized CDs from citric acid with glutathione for the sensing of mercury (Hg2+) ion. A high recovery of Hg2+ was achieved at 115.1%. The method of synthesizing CDs with low toxicity is embedded in the green chemistry principles. Thus, it fulfills the criteria of being eco-friendly. Table 2 provides a list of applications of CDs in the detection of heavy metals ions.
As seen in Table 3, the mechanism of action for the application of CDs largely depends on the analyte of concern. In the case of CDs from citric acid monohydrate for application in fluorescence quenching of Hg2+, it relies on Förster resonance energy transfer (FRET) [114]. This is similar to CDs synthesized from biomass [117], polyacrylamide [118], lotus root [119], degreased cotton [120], gold nanoclusters [111], and Petroleum coke [127].
Source of carbon nanoparticles | Sensing mechanism | Type of metal ions and linear range | Sensing (LOD) | Reference |
---|---|---|---|---|
Biomass | Fluorescence | Hg2+/Fe3+ 0.002 mol L−1 | 10.3 and 60.9 nM | [117] |
Polyacrylamide | Fluorescence | Hg2+ 0.25–50 μM | 13.48 nM | [118] |
Lotus plant | Fluorescence | Hg2+ 0.1 to 60.0 μM | 18.7 nM | [119] |
Degrease cotton | Fluorescence | Cr(VI) 1.00–6.00 mmol/L | 0.12 μg/mL | [120] |
Gold nanoparticles | Luminescence | Pb2+ 1 × 10−5 M | n/a | [121] |
Biomass from peanut shells | Fluorescence | Cu+2 0–5 mM | 4.8 mM | [122] |
Metal oxides | Electrochemical oxidation. | Cu2+ 0.1 to 1.3 μM | 0.04 μM | [123] |
Metal nitrates | Isotherm | Cd2+ 10 mg/L | 12.60 mg/g | [124] |
Mushroom | Fluorescent | Hg2+ 0 to 100 nM | 4.13 nM | [125] |
Penaeus merguiensis enzyme | Isotherm | Cu+2 1–5 mM | 2 mM | [126] |
Coke | Fluorescent | Cu+2 0.25–10 μM | 0.0295 μM | [127] |
Carbon nanoparticles for heavy metal sensing.
Fluorescent carbon nanoparticle sensing is largely dependent on changes or disturbances that are caused by an analyte that interacts with a fluorescent probe. This shift mostly will lead to a measurable change in the emission characteristics of the probe (emission wavelength, intensity, lifetime, or anisotropy), which can be directly linked to analytes (e.g heavy metal) concentration. More so, fluorescence probe strategies are based on quenching (turn-off) or enhancing (turn-on) emission, and surface-enhanced Raman scattering (SERS) techniques [33, 125].
Other notable techniques for the detection and quantification of heavy metals ions include, inductively coupled plasma mass spectrometry (ICP-MS). This instrumentation is efficient among several other methods, but it is expensive. It was developed since the 1980s [128, 129, 130], used mostly by multivariate analysis along with the ICP-MS technique to unravel heavy metal elements present samples. However, inductively coupled plasma atomic emission spectroscopy (ICP-AES) have also been used to identify heavy metal pollutants. But, the method is expensive and requires sophisticated instrumentations and a highly trained technician [131].
Nowadays, marine pollution is becoming a global phenomenon and seafood safety has played a crucial role in human health [129]. Fatema et al., [132] applied atomic absorption spectroscopy (AAS) to measure the absorbed quantity of Pb+2, Cd+2, and Hg+3 in shrimps. Heavy metals have been detected by other means such as energy dispersive x-ray fluorescence (EDXRF), electrothermal atomic absorption method (ETAAS), and flame atomic absorption spectroscopy (FAAS) [133, 134]. But, all of the aforementioned techniques have disadvantages in the detection of heavy metals, such that they are expensive and require strenuous experimental steps [135]. Therefore, environmental researchers have continued to strive to develop a cheap, simple, sensitive, specific, accurate, user-friendly, and eco-friendly means of detection for heavy metal pollutants.
CDs are very useful in detecting non-metallic elements. Several types of research have been reported, CDs synthesized from potato are well able to detect phosphate [106]. Zhaoxia et al., [136] utilized CDs with turnable emission and controlled size for sensing hypochlorous acid. As a class of carbohydrate that is widely distributed in a living organism, sucrose was chosen as a carbon source with assistance of microwave irradiation. A strongly fluorescent CDs without post-passivation was produced. By increasing the concentration of phosphoric acid as fluorescence enhancer under UV lamp, various fluorescent emissions of CDs of variable sizes were obtained. It was found that green CDs have excellent sensitivity for the detection of hypochlorous acid.
Kuo et al., [85], experimented with percutaneous fiber-optic nanosensors for instant evaluation of chemotherapy efficacy for in-vivo strategy of assay design aimed at monitoring non-homogeneously distributed biomarkers. They identified optimal exogenous fluorophores for the cell distribution indicators that are independent of the treatment of the apoptotic initiator and without interfering with the optical characteristics of fluorophores.
Huilin et al., [137], investigated on CDs as a fluorescent probe for off-on detection of sodium dodecyl-benzenesulfonate (SDBS) in aqueous solution. The pristine CDs were synthesized from sodium citrate through a simple, convenient, and one-step hydrothermal method. Fluorescent recovery was achieved with the application of SDBS. Detection of SDBS in real water samples was proportional to the concentration in the range of 0.10 to 7.50 ug/mL. Furthermore, fluorescence sensing probe has been used to detect kaempferol (flavonoid that is present in a variety of plants and plant-derived foods) using fluorescent CDs synthesized from chiefly acetic acid with a detection limit of 38.4 nM in the concentration range of 3.5–49 μM. Finally, organophosphorus as pesticides have been detected through the use of CDs as a detector for pollutants without surface modification [115].
CDs and carbon structured nanoparticles have attracted researchers to explore their effectiveness and optimization ability in the fields of pollution research [3]. Because of their excellent properties; carbon material performs concurrently as adsorbent and a transducing-agent [138, 139, 140, 141].
Due to abundant surface sites provided by CDs, it is a suitable candidate for studies in the detection and adsorption of heavy metals [142, 143]. For instance, Ghiloufi et al., [144] used gallium doped zinc oxide (ZnO) nanoparticle in the adsorption of heavy metals (Cd2+ and Cr6+) in aqueous solution. The adsorption of heavy metals was analyzed through the effect of pH and it revealed favourable adsorption at a low pH level, less than pH-3 and temperature of 298 K [144].
Table 4 provides harmonized presentation of nanomaterials applied for the purpose of absorbing environmental pollutants and contaminants in aqueous systems [145, 146, 147, 148, 149, 150, 151, 152, 153, 154].
Adsorbent material | Adsorbate/analyte | Reference |
---|---|---|
Gold nanoparticles (AuNPs) | 4-nitrophenol | [145] |
Carbon dots (sodium citrate) | Mercury (II) ions. | [146] |
Fluorescent carbon dots from o- phenylenediamine | Cell imaging and sensitive detection of Fe3+ and H2O2 | [18] |
Silica gel | Aromatic volatile organic compounds (VOCs) | [147] |
Graphene oxide | Nitrobenzene in sulfide | [148] |
TiO2, SiO2, and ZnO nanoparticles | Neptunium (V) | [149] |
Polystyrene latex nanoparticles | Alumina | [150] |
Graphene oxide | Radionuclide removal | [151] |
Polyaniline modified graphene oxide | Uranium(VI) | [152] |
Carbon nanotubes | Mingle-ringed N- and S-heterocyclic aromatics | [153] |
Graphene oxide | Minerals such as montmorillonite, kaolinite, and goethite, in aqueous phase | [154] |
Nanostructured materials in adsorption processes.
So far the concept of applying nanoparticles for environmental objectives have been successful. Meanwhile it is recommended that comparisons be made with bulk counterparts of the same substance to measure efficiency. On this note a study on the application of bulk agro material from jatropha curcas demonstrated efficiency in adsorption of pollutants and is recommended for comparison with its nano-dimension counterparts [155]. Similarly, a report on the application of sesame straw biochar in adsorption of heavy metal analyte concluded that further adsorption studies for nano-range agro-based materials are necessary for accurate estimation of adsorption in natural environments [156].
A suitable carbon source for CDs synthesis should be soluble in water (green chemistry), accessible worldwide (i.e. geographical abundance) with defined and well-known properties (i.e. functional attributes), should not be in direct competition with essential food production (i.e. sustainable), and it should be cost-effective (i.e. cheaply accessible). While the price of additives or carbon source plays a minor role in fundamental research, it may play a major role when large quantities are considered.
The authors would like to thank Universiti Putra Malaysia (UPM), Malaysia for funding this article.
The authors hereby declare that there is no conflict of interest.
M.Y.P., as the first author; made the study conception and design acquisition of reports and drafting of manuscript. Z.Z.A., contributed in the study conception and design, critical revision of major scientific ideas through clinical experience.
This research was funded by Universiti Putra Malaysia, grant number GP-IPS/2017/9556800.
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