Paper that apply supported AgNPs as heterogeneous photocatalyst.
The present chapter aims to overview the application of silver nanoparticles (AgNPs) in photocatalysis and biomedical field. Firstly, the relevance of AgNPs will be addressed. Then, the discussion about the photocatalytic activity of the AgNPs (either in suspension or impregnation), and correlation with your properties and its potential application to organic pollutants degradation under UV and visible/solar radiation will be described. Thus, applications of the AgNPs as antimicrobial agents, such as Escherichia coli, Schizophyllum commune, Staphylococcus aureus, Pseudomonas aeruginosa, Klebsiella pneumoniae, Haemophilus influenzae, Bacillus subtilis, Bacillus cereus and Enterobactor faecalis, and in the development of biosensors will be discussed. Therefore, the present work will be important to contextualize different scenarios to AgNPs mainly to wastewater treatment and diagnosis/therapeutic applications.
- metallic nanoparticles
- heterogeneous photocatalysis
- antimicrobial properties
Nanotechnology involves the manipulation of materials at nanometric scale (10−9 m) and have evolved to novel solutions for water/wastewater treatment as well as biomedical applications . These applications fields are possible due to the unique properties of nanomaterials, such as high surface area, high reactivity and considerable porosity and morphological, electrical, magnetic and/or optical properties, which turn them into useful materials in catalysis, adsorption, sensing and optic-electronic applications . Succinctly, nanomaterials can be divided into 2D, 1D, 0D, according to the number if dimensions the electrons are confined . Metallic nanoparticles are 0D materials, that is, they have the three dimensions within the nanoscale . Among metallic nanoparticles, silver nanoparticles (AgNPs) are largely investigated due to versatility in synthesis, easy processing, fast kinetic reaction rate, high thermal and chemical stability and so forth . Both related to water/wastewater treatment and biomedical applications, AgNPs features allows them to control the interaction with bacteria and, in the case of wastewater treatment, this control can be applied to several pollutants (dyes, hydrocarbons, pesticides, pathogens, and so forth) . Based on that, AgNPs can be applied to photocatalysis and to biomedical applications, such as antimicrobial agents and biosensing [7, 8]. Figure 1 shows the number of published papers involving AgNPs applied to photocatalysis and biomedical applications.
According to the Figure 1, it is possible to notice an increasing tendency of works related to application of AgNPs in the last 5 five years, mainly due to great efficiency of these nanoparticles in sensing applications.
2. Photocatalysis applications with AgNPs
2.1 Water quality deterioration
Mainly due to industrial expansion, climate change, population growth and anthropogenic activities, wastewater quality deterioration has been increased along the years . Wastewater contaminants can be either organic or inorganic . Therefore, the application of an adequate wastewater treatment technology has become a need for minimizing the pollution and the adverse environmental impact as well as for preserving the environment and attending to legal policies of water management .
Some of the inorganic ones are well known since ancient times such as chromium, copper, lead, nickel, cadmium, arsenic, mercury and others heavy metals. Theses pollutants pose a serious threat to human health and to environment due to their high toxicity . Meanwhile, organic contaminants, such as benzene, toluene, xylene and natural organic matter (measured by the dissolved or total organic carbon content), are of great concern wastewater management . Usually, the natural organic matter removal from wastewater is challenging and plays a crucial role in defining the treatment technology to be used .
In addition, emergent pollutants had been identified in wastewaters all around the world. Most of these contaminants have low biodegradability, high chemical stability, water solubility and are resistant against the conventional wastewater treatment processes . Pharmaceuticals and Personal Care Products (PPCPs) and organic dyes belong to this class of contaminants .
Similarly, bacteria commonly pose a threat to humans and to the ecosystem. Escherichia coli, Enterococcus faecalis, and Fusarium solani are the most contaminants found in wastewater and are of great public concern . Even though they are inactivated by conventional technologies (eg.: chlorination), secondary toxic pollutants are generally found after the treatment, which demonstrate the relatively low-efficiency of the traditional methods of disinfection . Therefore, sophisticated technologies for their inactivation are required, and at same time, it is expected that they do not result in secondary pollutants generation .
For these reasons, emergent pollutants (such as pharmaceuticals and dyes), as well as heavy metals, are difficult to remove from wastewater due to either the cost of sophisticated technologies or low efficiency in removing them . In this view, nanotechnology-enable processes seem to be promising to solve the wastewater deterioration quality problem . Therefore, the use of nanoparticles (iron, silver, titanium and zinc oxides) in wastewater applications has been increased due to the unique features of these nanomaterials, requiring treatments with relative low cost and reduction on labor time .
Advanced Oxidation Processes (AOPs) have proved to be highly efficient in the degradation of various contaminants like pharmaceuticals, dyes, hydrocarbons, pesticides, and pathogen [23, 24, 25]. AOPs are divided into two systems, being : (a) homogeneous, where there are no solid catalysts and the degradation of the organic compound can happen with or without irradiation (direct photolysis), using strong oxidizers (such as hydrogen peroxide, H2O2 and ozone, O3) with or without irradiation, such as photocatalytic ozonolysis and photo-Fenton and, (b) heterogeneous that are characterized by the presence of semiconductor catalysts with or without irradiation, such as heterogeneous photocatalysis and electro-Fenton. Among them, there are processes which are either based on the use of ozone or hydrogen peroxide, decomposition of hydrogen peroxide in acidic media or semiconductors such as titanium dioxide (TiO2) . The latter one is referred to as a heterogeneous process due to the ease of operation and to sustainable character. Also, it is promising in treating wastewater containing resistant contaminants, once a high percentage of refractory organic contaminants degradation can be achieved .
2.2 Heterogeneous photocatalysis
Heterogeneous photocatalysis is an Advanced Oxidation Process (AOPs) useful in the degradation of various contaminants, such as dyes, pharmaceuticals, pesticides, herbicides, hydrocarbons and microorganism inactivation. It can use either UV radiation or visible light to activate a metal-based photocatalyst (generally a semiconductor), promoting oxi-reduction reactions on the catalytic surface with considerable application in wastewater treatment . In this initial step, an electron moves from valence band (VB) to conduction band (CB) of the semiconductor and an electron–hole pair is generated (e−CB and h+VB) . Moreover, in this process the water molecules as well as the dissolved oxygen are used as precursors of the reactive oxygen species (ROS) generation .
Therefore, ROS (HO• and O2–•, mainly) tend to react non-selectively with the organic matter and to mineralize it altogether . Thus, heterogeneous photocatalysis is a light-induced catalytic process that reduces or oxidizes organic molecules through redox reactions, which are activated through the electron–hole pairs generated on the surface of the catalyst beyond band gap light irradiation, according to the Eqs. (1)–(8) :
Thus, the Eq. (1) represents the metal-based photocatalyst (semiconductor) activation yielding to electron–hole pair. Eqs. (2), (3), (6), and (7) are related to ROS generation, which are responsible for organic matter degradation. Eqs. (4) and (5) are undesirable recombination that takes place in the process. Meanwhile, Eq. (4) represents the spontaneous reaction of electron–hole pair recombination, which reduces the photocatalytic degradation efficiency, and Eq. (5) indicates hydrogen peroxide production. Eq. (8) holds for degradation of the organic matter by hydroxyl radicals. Thus, the great advantage of AOPs is that, during the treatment of organic compounds, they are destroyed and not just transferred from one phase to another, as occurs in some conventional homogeneous treatment processes.
Low human toxicity, high stability and relatively low cost are some of the advantages of the heterogeneous photocatalysis process . Moreover, there is a possibility of the complete mineralization of organic pollutants, generating CO2 and H2O . In this view, heterogeneous photocatalysis can be applied to degradation of a great number of recalcitrant and non-biodegradable organic pollutants [36, 37].
Moreover, this process can be combined with other processes (either pre-treatment or post-treatment), being performed in situ, making use of high oxidative potential agents with fast kinetic reaction and without to need for after-treatment or disposal . In addition, high-quality organoleptic characteristics of water can be achieved, with less energy consumption and low-cost of operation. It is important to mention that the absorbed energy by the semiconductor (and in its photoactivation) is related to the catalyst photocatalytic efficiency, which depends on the competition between the removal of the electron from the semiconductor surface and the recombination of electron–hole pair .
2.3 AgNPs-based supported photocatalysts
Individually applied to wastewater treatment, AgNPs proved to be extremely toxic to humans and aquatic life . Moreover, AgNPs can access various organs which most part of the substances cannot . For this reason, AgNPs have been associated to catalytic supports used in heterogeneous photocatalysis . In fact, AgNPs possess considerable photocatalytic activity and when they are impregnated in a less active material, labeled catalytic support, toxicity issues can be fixed . It is noteworthy that the main catalytic supports are metallic oxides, since they have favorable characteristics for applicability in heterogeneous photocatalysis, such as photoactivity within a UV–vis radiation range, redox potential of the positive conduction band high enough to promote the mineralization of the organic pollutant, high physical–chemical stability and efficiency in the oxygen reduction reaction. Table 1 shows some scientific papers found during the 2015–2020, which apply supported AgNPs as heterogeneous photocatalysis.
|Ag@MGO-TA/Fe3+||Methylene Blue (MB)||Excellent stability of the photocatalyst in the aqueous media, high reduction rate (0.054 s−1) for MB, under the dosage of 0.05 mg mL−1. 100% inactivation of E. coli using 20 μg.mL−1 of photocatalyst|||
|nAgFeO2||Imidaclopride (IMI)||80% degradation Imidaclopride under UV radiation|||
|Ag/wood||Methylene Blue and oil separation||The filter can separate selectively the oil from water and dye (about 99%). AgNPs incorporated to filter wood can improve the photocatalytic activity for MB degradation (94.03%)|||
|Ag@AgBr/Bi2MoO6||Reactive Blue-19||Excellent photodegradation of Reactive Blue 19 (98.7%) under visible light after 120 minutes|||
|C@CoFe2O4@Ag||Red and Methyl Orange||Fast kinetic of degradation (7 min) for Red Orange and for Methyl Orange (10 min)|||
|Ag/ZnO/PMMA||Methylene Blue, Paracetamol and Sodium Lauryl Sulfate||90% degradation for the target pollutants after 4 hours under UV radiation|||
|Ag@CAF and Ag@TiO2||4-nitrophenol (4-NP), 2-nitrophenol (2-NP), 2-nitroaninile (2-NA), trinitrophenol (TNP), Rhodamine B (RhB) and Methyl Orange (MO)||Nanomaterials exhibited high photocatalytic activity (95%) efficiency was achieved after 10 min|||
|glass/Ag||Textile wastewater||The coat glass (with AgNPs) yields to about 95% of dye removal after 5 h. In addition, the reusability was studied, targeting microbes found in wastewater. After 2 h, 80% of microbes were inactivated|||
|Chitosan/Ag||Sodium Fluoresceine||48% dye degradation under anaerobic condition after 2,5 hours. About 51% degradation was achieved under aerobic condition. Chitosan/AgNPs nanocomposite showed antimicrobial activity for gram-positive (E. coli) and gram-negative bacteria (G. bacillus).|||
|Fe3O4@PPy-MAA/Ag||4-nitrophenol (4-NP), Methylene Blue and Methyl Orange||Excellent catalytic activity towards 4-Nitrophenol, Methyl Orang and Methylene Blue. Good reusability of the nanophotocatalyst|||
|rGO-Ag||MB and RhB||Degradation of 95% is observed, which is significantly greater that the pristine nanophotocatalyst AgNPs (78%) and rGO (55%)||[53, 54, 55]|
In addition, the main drawbacks of the metallic nanoparticles, such as nanoparticle agglomeration, can be solved with the application of the Ag-based supported photocatalysts . Thus, impregnation of AgNPs onto ZnO photocatalyst results in better photocatalytic activity, when compared to the use of unsupported ZnO for degradation of Methylene Blue dye. In addition, associations of AgNPs to AuNPs in a core-shell structure leads to a photocatalytic activity comparable with the commonly used TiO2 photocatalyst. As can be seen, about 80% Methylene Blue degradation can be achieved, when AgNPs supported onto bismuth vanadate (BiVO4) are used .
Moreover, with respect to inactivation of bacteria, efficient inactivation degrees for E. Coli, F. Solani, and E. faecalis are reported by means of heterogeneous photocatalysis . Additionally, the efficiency of supported AgNPs for inactivating some pathogens in real wastewaters has been confirmed, resulting in about 80% inactivation .
Meanwhile, the utilization of AgNPs as for potentializing the metal-based catalyst and others materials applied to the degradation of organic pollutants or the discoloration of the wastewater is considered an efficient technology, accounting for up to 90% after 180 minutes under UV radiation.
3. Biomedical applications with AgNPs
Silver nanoparticles have been the most investigated, among other metallic nanoparticles for biomedical applications, such as antimicrobial agents and biosensing due their unique physicochemical and biological properties . However, the effective application of AgNPs in the biomedical field is strictly related to their morphology, that is, size and shape .
Moreover, one of the main applications of AgNPs, within the biomedical area, consists of acting as an antimicrobial agent, capable of inhibiting the growth of pathogenic microorganisms, being indicated for the treatment of bacterial infections . There are several proposed mechanisms that explain the antimicrobial activity of AgNPs, and all of them lead to applications in wound healing , bone tissue , and surface coatings . Besides that, optical, electrical and plasmonic properties of AgNPs turn them into interesting nanostructured materials to be used in chemical and biological sensing .
Despite being effective against several pathogens and showing promising potential in biosensing, safety concerns are yet a challenge nowadays . In spite of their outstanding properties to biomedical applications, it is known that AgNPs can be toxic to humans depending on the concentration  or due to toxic chemicals involved during the synthesis process . To overcome this, nanotechnology has been used together with green chemistry , leading to the synthesis of AgNPs by using alternative sources/materials, such as plant extracts, biopolymers and microorganisms (e.g. bacteria and fungi) . Thus, particular interest in evaluating the biocompatibility of green-synthesis derived AgNPs have also been attracting the attention of the scientific community . In the following subsections, both AgNPs applications as antimicrobial agents and as biosensors are discussed.
3.1 Silver nanoparticles as antimicrobial agents
The pathogenic microorganisms are constantly evolving, with a wide genetic diversification, being capable of eliciting several diseases . Although, there are various antimicrobial therapies commercially available, the use of conventional therapies led to the gain of resistance by these pathogenic bacteria .
The emerging resistance of bacteria against the conventional antibiotics and metallic ions increased the research in the field of applied nanotechnology, with the AgNPs being among the most potent compounds due to their high specific surface area and number of atoms available to interact with the surroundings, resulting in exhibiting unique properties (electronic, bactericide, magnetic, and optical), since these favorable properties favor the generation of reactive oxygen species (ROS), which they cause changes in the structure of proteins and nucleic acids, and in the permeability of the cell wall, culminating in the lysis of the bacterial cell .
The biosynthesis (both intracellular and extracellular) uses microorganisms and show advantages compared to chemical processes: (i) easy strain manipulation, supporting the synthesis process, (ii) easy scaling-up, and (iii) no generation of toxic substances to the environment. However, as the main disadvantages of the biosynthesis using microorganisms is the need to use of the ultrasound to unbind the AgNPs [76, 77].
It is known that the AgNPs have great potential against several gram-negative, gram-positive and antibiotics-resistant bacterial strains . The antimicrobial activity depends on the nanoparticle size and concentration, where low particle sizes with low concentrations can kill bacterial strains and, in the case of green-synthesis derived nanoparticles, this is allied to the advantage of showing lower toxicity to human health and to the environment, leading to a high interest in developing them to combat pathogenic bacteria .
The antimicrobial activity of AgNPs against pathogenic bacteria follows two action mechanisms: according to the first one (i), the nanoparticles adhere to the cellular membrane and penetrate the bacteria, promoting alterations on their cellular membrane (due to interactions of silver ions with proteins, sulfur, and phosphorous within the cell, avoiding the electron transport) and, then, resulting in bacterial growth suppression ; the other mechanism (ii) involves the silver ions releasing and the production of reactive oxygen species which generates an antimicrobial effect .
Moreover, the bacteria are generally unable to develop resistance against AgNPs, which are specially formulated due to their particle size and that allows them to attack a wide range of targets present in the microorganisms, such as proteins, thiol groups and cellular walls . In fact, AgNPs have a huge potential to be used as antimicrobial agents depending on the physicochemical characteristics of these nanoparticles. Therefore, the synergistic effect of these properties, associated with low production cost, good thermal and radiation stability (UV and visible), doing AgNPs effective against pathogenic microorganisms, being promising in biomedical applications to reduce infections. Table 2 shows the different results of AgNPs against several pathogenic bacteria.
|Green AgNPs||Escherichia coli and Schizophyllum commune||Green synthesis using Eucalyptus camaldulensis (EC). Moreover, High antimicrobial activity due to the high surface are, which supports better cellular interactions with pathogenic microbes|||
|Green AgNPs||Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa, Klebsiella pneumoniae and Haemophilus influenzae||Green synthesis using Artemisia vulgaris leaves extract (AVLE). In addition, antimicrobial activity with mechanisms involving penetration in the bacteria and silver ions releasing, leading to bacterial growth suppression.|||
|Green AgNPs||Bacillus subtilis, Bacillus cereus and Staphylococcus aureus||Green synthesis using Acorus calamus rhizome extract. Moreover, nanoparticles exhibited antimicrobial activity when adhering to the bacterial cellular membrane, inhibiting the cellular growth|||
|Green AgNPs||Escherichia coli and Staphylococcus aureus||Green synthesis using Vitex negundo L. AgNPs showed antimicrobial activity against both gram-positive and gram-negative bacteria|||
|AgNPs||Escherichia coli and Staphylococcus aureus||Synthesis by chemical reduction and by using Petroselinum crispum plant, and showed antimicrobial activity by penetrating the cellular wall of bacteria|||
3.2 Silver nanoparticles applied to biosensing
AgNPs have been investigated for chemical and biological sensing. It was already found in literature that the AgNPs present better results than gold nanoparticles (AuNPs) when related to biosensor sensitivity, despite the AuNPs being more investigated for biosensing applications . In addition, AgNPs are plasmonic nanostructures, which means that they can absorb and scatter light . Thus, AgNPs could be used to provide a colorimetric/plasmonic method to detect several biomolecules (including eye-naked verification) due to their light absorption bands - around 400–500 nm  - being within the visible range of the electromagnetic spectrum. It is worth pointing out that there are two types of colorimetric/plasmonic biosensors: the aggregation-based and the LSPR-based  - some of them are summarized in Table 3. Nevertheless, control over morphology during the synthesis of AgNPs is crucial, as anisotropic AgNPs can display various light absorption bands rather than just one .
|AgNPs-based SPR biosensor||MicroRNA (miRNA) detection||LSPR-based||Good sensitivity and selectivity. Excellent reliability. No need for modification procedures to amplify biosensing|||
|AuNPs/AgNPs biosensor||Cyclin A2||Aggregation-based||Simplicity, high sensitivity, and selectivity. Eye-naked verification allied to quantitative detection. No need for functionalization of the AuNPs/AgNPS. Suitable for peptide-based protein detection. Detection limit: 30 nM.|||
|Citrate capped silver nanoparticles (Cit-AgNPs)||Acinetobacter baumannii detection||Aggregation-based||Accurate and quick detection (about 2 min); low detection limit of quantification (LLOQ) of 1zM.|||
|Carbon quantum dots (CQDs)/AgNPs nanocomposite||Melamine detection||Aggregation-based||High sensitivity, simple method of detection, and eye-naked verification allied to quantitative detection. Detection limit: 65.3 pmol.L−1.|||
|Glutathione-coated AgNPs||Vitamin B1 (thiamine) detection||LSPR-based||Provides both qualitative (colorimetric) and quantitative sensing. High sensitivity and selectivity; low-cost, quick, and simple detection. Worked well with real samples, such as blood and urine.|||
|Grown-AgNPs on AuNS||Alkaline phosphatase (ALP) detection||LSPR-based||Provides both qualitative (colorimetric) and quantitative sensing. Could be extended to a general device for immunosensors/aptasensors designs.|||
|SiOx/AgNPs/Graphene||DNA hybridization detection||LSPR-based||Other applications may involve enzyme detection, medical diagnostics, food safety, and environmental monitoring. Sensitivity improvement of 304.60% compared to pure AgNPs.|||
|Ag@AgCl nanotubes loaded onto reduced graphene oxide (RGO)||Ochratoxin A (OTA) detection||LSPR-based||Good accuracy, high sensitivity, and good reproducibility. High stability and photocurrent response under visible light irradiation. Range of detection: 0.05 to 300 n mol L−1, limit of detection (LOD): 0.01 n mol L−1 (4.0 pg. mL−1).|||
|AgNPs-based aptasensor||Adenosine detection||LSPR-based||High sensitivity and selectivity. Simple and specific design, low-cost, and quick detection. Linear range of detection: 200 n mol L−1 to 200 μ mol L−1, detection limit: 48 n mol L−1.|||
Moreover, Localized Surface Plasmon Resonance (LSPR) phenomenon of AgNPs have turned them into interesting nanomaterials for applications, which involve interaction with light . Thus, when a metallic nanoparticle is irradiated, superficial electrons oscillate collectively, and these generate an electromagnetic field around the nanostructure, called Surface Plasmon Resonance (SPR) . If an external electromagnetic field is applied, in such a way that it is in resonance with the generated electromagnetic field around the metallic nanoparticle, LSPR phenomenon takes place . Thus, the LSPR is possible due to the confinement of the resulting electromagnetic field within the metallic nanoparticle [104, 105].
Moreover, the aggregation-based AgNPs biosensors are considered low-cost, and high-sensitivity biosensing devices, as the aggregation of AgNPs depending on induced changes on the solution medium can be applied to detect DNA molecules, proteins (recognizing) . In this case, aggregation phenomenon and chemical instability of AgNPs is desirable, once it is the working principle of the biosensor . LSPR-based ones are established on changes in the occurring refractive index now that photons are directed to the nanoparticles, leading them to oscillation , being used in biomolecules detection . It is also important to mention that AgNPs are normally functionalized before applying them as biosensors to overcome chemical stability and toxicity aspects . Therefore, coating AgNPs with organic or inorganic materials are the common approaches. Furthermore, polymeric coatings are also used to functionalize AgNPs by using either synthetic polymers, such as (poly)-ethylene glycol (PEG) , (poly)-vinyl alcohol (PVA)  and (Poly)-vinylpyrrolidone (PVP) , or natural polymers such as starch , sodium alginate  and chitosan . Functionalization with polymeric blends that uses both synthetic and natural polymers is also an interesting approach (e.g. PVA/Chitosan-coated AgNPs ). The inorganic coating involves the functionalization of AgNPs with silicon dioxide , while organic coating involves citrates mainly . Furthermore, there are plenty of electrochemical-based AgNPs biosensors [120, 121], however, they will not be covered here as the focus is on the plasmonic ones. Another possible application of AgNPs is the surface enhanced Raman Spectroscopy (SERS), which involves the adsorption of molecules on the AgNPs to achieve a high-quality spectroscopy technique. The applications of SERS focus on disease diagnosis caused by microbial infections or cancer .
Therefore, AgNPs-based biosensors are good alternatives against conventional sensing devices, as nanostructured biosensors show greater sensibility, reliability, wide limits of detection, precision, speed and provides eye-naked colorimetric assays together with quantitative analysis , among other unique characteristics that are shown in the papers summarized in Table 3.
Regarding the use AgNPs in heterogeneous photocatalysis, it can be proved highly efficient in the degradation a large amount of organic pollutants and inactivation of bacteria and pathogens, under either UV radiation or visible light. Moreover, when supported AgNPs-based nanophotocatalysts are used in wastewater not only the photocatalytic activity is enhanced, but also some operational problems (nanoparticles agglomeration) can be fixed. With respect to the use of AgNPs as antimicrobial agents, it is a current alternative against common pathogens and multi-resistant bacteria due to the toxicity to microorganisms compared to antibiotics and conventional approaches. In addition, AgNPs-based biosensors are resulting in high sensitivity and selectivity aligned to wide detection limits, which turns them suitable for clinical practice. It is worth to point out that the green synthesis of AgNPs is increasing along the years, and when combined with photocatalytic and biomedical applications, contributes to sustainable development and biocompatibility aspects.
This work received financial support from the Foundation for Research of the State of Rio Grande do Sul (FAPERGS – Project 19/2551-0001362-0), so all thanks.
Conflict of interest
The authors declare no competing interests.