Surface-Enhanced Raman Scattering Liquid Sensor for Quantitative Detection of Trace Melamine in Dairy Products

2.1. Optimization of the surface-enhanced Raman scattering liquid sensorfor melamine detection

In recent years, gold nanoparticles (Au NPs) and silver nanoparticles (Ag NPs) have been widely used as colorimetric probes for chemical sensing and biosensing of various substances (Zhao, et al., 2008), such as viruses (Niikura et al., 2009), protein (Wang et al., 2008), DNA (Cho, et al., 2008), cancerous cells (Medley et al., 2008), and small molecules (Chen, et al., 2010; Li et al., 2009; Zhang et al., 2008), relying on their unique size-dependent and/or interparticle distance-dependent absorption spectra and solution color. For example, triple hydrogenbonding recognition between melamine and a cyanuric acid derivative grafted on the surfaced of Au NPs can be used for reliable detection of melamine (Ai et al., 2009).

Currently, much attention has been paid to the study of the optical absorption spectra of nanoscale colloidal silver in the quest for SERS enhancement factors. Compared to Au NPs, Ag NPs have some advantages, for example, lower cost of preparation and higher extinction coefficients relative to Au NPs of the same size (Lee, et al, 2007). Therefore, Ag NPs are also good candidates for melamine sensing (Han, et al., 2010; Ping et al., 2012).

Upon considering the influence of temperature, ionic strength, and aggregation behavior of colloids on the SERS spectra band intensity in the presence of adsorbates and the wavelength at which maximum enhancement occurs, the latter shift to higher values with time. In particular, the adsorption of the colloid is strongly influenced by chloride ions (Koglin et al., 1996) and pH (House et al., 2008). Scanning electron microscopy images of a silver colloid before and after addition of reagent A (Sodium chloride aqueous solution or aqueous potassium chloride solutions) and reagent B (Aqueous sodium hydroxide or potassium hydroxide solution) are presented in Figure 1. As shown in Figure 1a, the colloidal silver particles mainly displayed a spherical morphology with a uniform size of ~70-100 nm. After added reagent A (Fig. 1b) or reagent B (Fig. 1c), the silver colloid became aggregated and inhomogeneous. When reagents A and B were added to the colloidal silver at the same time, the morphology of the silver colloid became more dense and uniform (Fig. 1d), which is the best form for SERS enhancement. Thus, this system was chosen as the surfaced-enhancing substrate for further study. Figure 1e shows the SERS spectra of 1 μg mL^-1 melamine on the corresponding enhancing substrates from (a), (b), (c) and (d) in Figure 1. There are no evident Raman bands of melamine on silver colloid (curve ⅰin Fig. 1e) or on silver colloid with reagent A (curve ⅱin Fig. 1e). However, when reagent B was added to the silver colloid (curveⅲin Fig. 1e), a weak characteristic peak of melamine was observed at 698 cm^-1, i.e., the SERS spectra band intensity was affected by pH. After reagents A and B were added to the silver colloid (Curve IV in Fig. 1e), the characteristic peak of melamine was strongly enhanced, with the intensity of the peak at 698 cm^-1being the greatest.

2.2. Description of use of the milk melamine liquid sensor

It is believed that melamine (2,4,6-triamino-1,3,5-triazine) is sometimes intentionally added to food ingredients to make the products appear to contain higher protein levels due to the high nitrogen content of melamine. A safety limit for melamine ingestion is officially set at 2.5 ppm for adult food and 1 ppm infant formula by the US Food and Drug Administration (Zhao et al., 2009; http://www.fda.gov/NewsEvents/ Newsroom-/ PressAnnouncements/ 2008/ ucm116960.htm.). The maximum residue level of melamine in infant formula is now legally regulated at 1 ppm by the Chinese government after the recent melamine accident (Guo et al., 2010). To achieve this lower limit of detection (LOD), silver colloids are ideal candidates to be used as surfaced-enhancing substrate liquid sensors due to their strong Raman-enhancing effect. Thus, we chose silver colloid as a surfaced-enhancing substrate for the detection of melamine in this study, and the detection process is diagrammed in Figure 2.First, liquid milk was diluted with double-distilled water (Fig. 2a) toobtain a diluted milk sample. Next, the diluted sample was placed into a 1.5-mL conical centrifuge tube and centrifuged for 4 min at 14,000 rpm, and then it was delaminated (Fig. 2b). Next, the supernatant was removed from the centrifuge tube and was added to the silver colloid, which was previously prepared with drop-wise addition of reagents A andB, and uniformly mixed (Fig. 2c).Finally, the SERS spectra were recorded using a portable Raman spectrometer (Fig. 2d) to collect analytical results.

2.3. Optimization of the melamine spectra

Based on these experimental results, the spectra of different concentrations of melamine in solution were investigated from 500–1200 cm^-1, as shown in Figure 3.Typical Raman peaks of solid melamine at 382, 584, 678, and 983 cm^-1 were observed (Fig. 3a).The most intense peak at 678 cm^-1 is assigned to the ring breathing II mode, which involves in-plane deformation of the triazine ring. And the second most intense peak at 983 cm^-1 arises from the ring breathing mode I of the triazine ring (Koglin et al., 1996). The peaks at 698 and 1005 cm^-1, visible in the SERS spectra of Figure 3b–d, were obtained from melamine samples at concentrations of 5×10^-1, 10^-1, and 10^-2 μg mL^-1. The Raman spectra of the enhanced substrate, i.e., silver colloid treated with reagents A and B,is shown in Figure 3e.In the absence of melamine, small peaks at 698 and 1005 cm^-1 were observed, and the other peaks disappeared. Only a small peak at 678 cm^-1 was observed in the Raman spectra of melamine dissolved in water (Fig. 3f), and no peaks were evident in the spectra obtained from the 10³ μg mL^-1 melamine sample in the absence of the enhancing substrate (Fig. 3g).

Clearly, in the SERS spectra, the relative intensities of the peaks at 698 and 1005 cm^-1 were enhanced with increasing melamine concentration. Furthermore, the spectra markedly changed in the presence of the silver colloid enhancing substrate. The specific melamine peaks at 698 and 1005 cm^-1 were shifted by 20 cm^-1, which is a large Raman shift compared to the peaks for the Raman spectroscopy of solid melamine. This may be due to the effect of the enhancement system (Haynes et al., 2005). In addition, due to “electromagnetic field enhancement” and “chemical or electronic enhancement” (He et al., 2008; Kneipp et al., 2002), Raman signals can be significantly enhanced, by up to six orders of magnitude, in the highly localized optical fields of such structures. Therefore, this specific approach is reasonable and promising for the field detection of melamine in various liquid milk products, such as raw milk, pure milk, and yoghurt.

2.4. Analysis of detection results

To demonstrate the practical application of melamine in liquid milk, we used melamine in raw liquid milk as an example. Various concentrations of melamine in liquid milk were extracted and analyzed by their SERS spectra (Fig. 4). As shown in Figure 4a, seven concentrations (0.5, 1, 2, 2.5, 5, 8, and 10 μg mL^-1) of melamine in liquid milk were studied, and the intensity of the melamine peak at 698 cm^-1 was enhanced with increasing melamine concentration. To eliminate the effects of the matrix and other factors (e.g., temperature, humidity, and focal distance), the intensity of the peak at 928 cm^-1 was set at 100 for milk, and the Raman peak at 698 cm^-1 in the absence of melamine had a fixed value. Accordingly, a melamine standard curve was obtained by establishing a plot correlating the melamine concentrations in liquid milk to the intensity of the intense SERS spectral peak of melamine at ~698 cm^-1. A linear regression (R² = 0.9996) was found between the Raman intensity and melamine concentration (Fig. 4b). The limit of quantification (LOQ) using this approach to detect melamine in liquid milk was also investigated, as shown in Figure 5. We found that this specific approach is reasonable for the detection of melamine in liquid milk because only one prominent peak was present in the melamine SERS spectra, which can be applied to field detection of various liquid milk products.

Moreover, the tests were performed and assessed by the Ministry of Science and Technology of the P. R. China, complying with the general administration quality supervision inspection quarantine of the P. R. China, the Ministry of Agriculture of the P. R. China, the Ministry of Health of the P. R. China, and the National Institute of Metrology P. R. China. The SERS test results were very precise and as good as those obtained by the LC/MS/MS method (Table 1). Forty-nine of 50 test samples results were correct, i.e., melamine was correctly detected in 98% of the test samples (Table 2).The concentration error in the samples was 0.2 ppm, which exceeds the limit of quantification using Raman spectra. The relative standard deviations (RSDs) were ≤ 10%, and the relative measurement deviations (RMD) were ≤ 10%.Therefore, the SERS method is an effective approach for measuring liquid milk melamine, which provides on-line, rapid, and reliable screening.

A method was established to detect melamine in liquid milk using surface-enhanced Raman spectroscopy with the aid of a silver colloid enhancing substrate. An enhancement factor of ≥ 10⁵-fold was achieved in the measurement of melamine on this SERS-active substrate. In addition, the milk sample preparation process used in this technique is easy and time-saving, only requiring four steps: dilution, centrifugation, addition of samples to the enhanced base, and collection of the Raman spectra. The total detection time using SERS to measure a sample was ~3 min, which is starting from the dilution up to the final results. And the Raman spectra were acquired for only 3 s. Based on the calculations of the most intense peak in the melamine SERS spectra at approximately698 cm^-1, the LOQ of the SERS spectra achieved a level of 0.01 μg mL^-1 for melamine standard samples, which corresponds to 0.5 μg mL^-1 melamine in liquid milk. The RSD was ≤ 10 %, and recoveries were from 93-109%.The results from actual sample analyses were very precise and as good as those results obtained by LC/MS/MS.