Diethyldithiocarbamate (DDTC) induced formation of positively charged silver nanoparticles for rapid in situ identification of inorganic explosives by surface enhanced Raman spectroscopy

Abstract

The nonvolatile inorganic explosives deposition on various surfaces requires rapid in situ detection by homeland security. Surface enhanced Raman spectroscopy (SERS), a prompt and sensitive detection method, has been widely applied in chemical and biological samples analysis. The most popular noble metal nanoparticle colloids, prepared by chemical reduction with citrate or hydroxylamine, are generally negatively charged. This study demonstrates that the negatively charged silver nanoparticles were transformed into the positively charged ones by sodium diethyldithiocarbamate (DDTC), a potent copper chelating agent. The zeta potential of DDTC modified silver colloid is about 32.1 mV, suggesting its positively charged nature. The XPS and SERS spectra indicate the positively charged silver nanoparticles resulted from the bidentate configuration leading to the redistribution of the free electron pair in N atom and formation of the C=N⁺ bonding of DDTC. The positively charged silver nanoparticles can be used in the detection of inorganic explosives anions with a subnanogram detection limit through electrostatic interactions. The common organic explosive picric acid (PA) could also be detected through a similar interaction force.

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1. Introduction

Security issues have caused widespread concern with the increase of the globalization of terrorist acts over nearly two decades.^1,2 Inorganic explosives, consisting of an inorganic oxidizer such as sodium nitrate (NaNO₃), or sodium perchlorate (NaClO₄) and a fuel such as carbon source, sulphur (S), sugar or powdered metals, have been employed very frequently because of the easy acquisition in contrast to the strictly controlled organic high explosives.^3–8 The regulation of explosives is more important like finding hidden explosives deposited on various package surfaces such as synthetic fabric bags and rubber materials.^9,10 Hence, in a wide variety of scenarios, in situ detection technology of trace explosive deposited on various surfaces is in high demand for homeland security needs.^9,11,12 Currently, the traditional methods of detection of inorganic explosives are commonly performed complex instrumentation such as ion mobility spectrometry (IMS),¹³ capillary electrophoresis (CE),¹⁴ ion chromatography (IC)^15,16 and electrospray ionization mass spectrometry (ESI-MS),¹⁷ which required the sampling methods by swab sampling for explosive particulates. Hence, the rapid in situ detection of nonvolatile inorganic explosives on the surface of suspicious objects still remains a challenge.^18–20

Raman spectroscopy has been shown to be a highly efficient tool for the identification of a variety of samples since it is rapid, noninvasive and nondestructive, provides confirmatory identification of analyzed samples.²¹ But ordinary Raman signals are typically too weak to be reliable for the identification and quantification of molecules.²² Fortunately, Raman scattering can be greatly enhanced when the molecules adsorb on a rough metal surface, a phenomenon generally called surface-enhanced Raman scattering (SERS).^23–26 Due to the fine sensitivity, selectivity and fingerprint characteristics, SERS is a widely pursued spectroscopic tool in the analysis of chemical and biological samples,^27–30 e.g. the determination of surface molecules at materials or biological tissue,^31–33 the detection of pesticide residues in agricultural products,^34,35 and the probe of explosive particulates.^36–38 The metal nanoparticle aggregates in colloids has been widely exploited as SERS enhancing substrates due to the combination of the high enhancement factors that aggregated nanoparticles provide and the simplicity of colloid preparation, it leads to low analysis costs and performs in situ detection.^39–41 The most popular noble metal nanoparticle colloids are generally prepared by chemical reduction of an appropriate soluble metal salt,^42–44 e.g. silver^45,46 and gold⁴⁷ colloidal nanoparticles charge-stabilized by anions such as citrate and hydroxylamine with negatively charged nature,⁴⁸ which can be usually employed to detect the compound with sulfur or nitrogen such as the in situ detection of hydrazine in the lake water⁴⁹ and the detection of pesticide at fruit peels³⁴ and so on.

In this study, the transformation of the negatively charged silver nanoparticles colloid into positively charged ones was demonstrated by sodium diethyldithiocarbamate (DDTC), a potent copper chelating agent. The morphology and mechanism of DDTC inducing positively charged silver nanoparticles were then investigated. The positively charged silver nanoparticles colloid was applied in situ to detect the inorganic explosives anions NO₃⁻ and ClO₄⁻ on the surface with a portable Raman spectrometer through the electrostatic interaction.

2. Experiment

2.1. Chemicals

Silver nitrate (AgNO₃, 99.8%), trisodium citrate dihydrate, sodium diethyldithiocarbamate (DDTC), ethanol (chromatographic grade) were all obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Sodium perchlorate (NaClO₄·H₂O, 99.0%) were purchased from Aladdin Chemicals. Picric acid (PA, 99.0%) was obtained from TCI Development Co., Ltd. (Shanghai, China). Sodium nitrate (NaNO₃, 99.0%) was purchased from Guangcheng Chemical Reagent Co., Ltd. (Tianjin, China). Tin foil was purchased from super market. All the reagents were used without further purification. Analytes except for picric acid were dissolved in ultrapure water (18.25 MΩ cm) to obtain the 10⁻² M solution for further use. For picric acid, the solvent was ethanol.

2.2. Characterization techniques

Transmission electron microscopy (TEM) (JEM-1011, JEOL, Japan) with an acceleration voltage of 100 kV was used to characterize the silver colloid. A scanning electron microscope (SEM) (JSM-6700F) was used to characterize the morphology of the silver colloid on the tin foil. The ultraviolet-visible (UV-Vis) spectrum of silver nanoparticles was collected on a Tu-1901 spectrophotometer from Beijing Purkinje General Instrument Co., Ltd. The size distribution was obtained by dynamic light scattering (DLS) (BI-200SM, Brookhaven, USA) and counted by Continsortware. Zeta-potential was measured by laser Doppler electrophoresis, using a Malvern Zetasizer Nano ZS90 equipped with a 633 nm laser. An Ocean Optics QE65000 Raman spectrometer equipped with a TE-cooled FFT-CCD detector was operated throughout to measure the SERS signal on the DDTC-modified silver colloid. The excitation wavelength was 785 nm, the output laser source of the instrument was operated at 250 mW. The laser focus was about 158 μm, the actual power density was less than 0.01 mW μm⁻² and the total accumulation time of the SERS measurement was 1 s. A model pHS-3C digital pH meter (Shanghai, China) was served to measure the pH values.

2.3. Preparation of silver colloid and DDTC modified silver colloid

Silver colloid was synthesized according to Lee and Meisel's method.⁴⁵ In brief, 0.018 g silver nitrate powder was dissolved in 100 ml water and heated until boiling. Then 2 ml of 1% trisodium citrate aqueous solution was added. The solution was kept boiling for half an hour, accompanying an obvious colour change from less yellow, orange to brown and greyish-green, successively. The resulted colloid was cooled at room temperature for further used. The positively charged silver colloid was prepared by modifying with DDTC. 10 ml of silver colloid was added to 350 μL of 1 × 10⁻⁴ M DDTC solution dropwise with vigorous stirring to produce the positive colloids. The DDTC modified Ag NPs colloid was centrifuged and dispersed into 0.5 ml of DDI water to remove the surplus DDTC.

2.4. Sample detection

Sample was prepared at 10⁻² M in DDI water, then dilution to the desired concentrations in ethanol. 10 μl of each test solution was dropped into the reaction vessel, which was made by overlapping the cover of 96-well plate with tin foil. Subsequently, 20 μl concentrated DDTC-Ag NPs solution was mixed with the test solution, then dried under ambient conditions. The SERS spectrum of sample was measured from the DDTC modified Ag NPs.

3. Result and discussion

3.1. Morphology and zeta potential analysis of the DDTC induced positively charged silver nanoparticles

The morphology of the silver nanoparticles before and after modification was investigated. As shown in Fig. 1a and b, both citrate-Ag NPs and DDTC modified Ag NPs display uniform spherical morphologies. The UV-Vis spectroscopy (Fig. 1c) shows that the surface plasmon resonance (SPR) peak of DDTC modified Ag NPs (434 nm) is retained but shows a slight red shift of 2 nm compared to that of citrate-Ag NPs. The DLS (Fig. 1d and e) indicates the average hydrodynamic size of citrate-Ag NPs is 71 nm. The average hydrodynamic size of DDTC modified Ag NPs is reduced to 67 nm due to the DDTC replaced the surface citrate ions in silver nanoparticles. The concentration of the Ag NPs is 0.1 nM, calculated using Beer's law and extinction coefficients (ε) of 7.2 × 10¹⁰ M⁻¹ cm⁻¹ for Ag NPs.⁵⁰ These results suggest that the modification of DDTC does not affect the morphology and dispersed state of Ag NPs. In addition, the zeta potential (Fig. 1f) of DDTC-modified silver colloid is about 32.1 mV, comparing to −43.7 mV of original silver colloid, which suggest that the DDTC induced the surface charge conversion of the silver nanoparticles.^51–54 The positively charged silver colloid could keep the size and dispersity.^55–57

Fig. 1 The morphology of silver nanoparticles colloid before and after DDTC modification. (a) TEM images of Ag NPs and (b) DDTC modified Ag NPs. (c) UV-Vis extinction spectra of Ag NPs solution and the DDTC modified Ag NPs solution. (d) The size distribution of Ag NPs. (e) The size distribution of DDTC modified Ag NPs. (f) ζ potential distribution for silver colloid (red) and DDTC-modified silver colloid (green).

3.2. Spectral characterization of the DDTC induced positively charged silver nanoparticles

DDTC, a classical complexing agent, is often utilized as copper reagent in molecular absorption spectrophotometry.⁵⁸ In this work, the DDTC is used to modify the silver nanoparticles. As shown in Fig. S1a,† the presence of the peak on the EDS spectrum for S element indicates that the DDTC is adsorbed on the surface of the silver nanoparticles. The XPS characterizes the binding energy of Ag (3d) for the silver colloid before and after DDTC modification in Fig. 2a. The fact that the binding energy of Ag (3d_5/2) shifts to a lower value by 0.45 eV and the Ag (3d_3/2) shifts to a lower value by 0.42 eV indicates the interaction of DDTC with silver nanoparticles and the formation of the Ag–S binding.^59,60 In addition, the binding energy of S (2p) at 162 eV after modification could be attributed to a normal S–Ag bond, which is consistent with the previous literature (Fig. S1b†).⁶¹ The Raman spectra of the DDTC-modified silver colloid and DDTC powder is shown in Fig. 2b. The characteristic Raman bands of DDTC onto the silver colloid appear at 1273 cm⁻¹, 1079 cm⁻¹, 1343 cm⁻¹, 1453 cm⁻¹, and 1497 cm⁻¹, which are consistent with the previous report.⁶² The Raman spectrum peak at 1497 cm⁻¹ is assigned to the υ_as(C=N) + δ(HCH)(CH₂) sciss.⁶³ After modifying on the silver nanoparticles, the Raman peaks of the DDTC powder at 310 cm⁻¹ (the N–C=S in plane bending) and 423 cm⁻¹ (the S=C–S stretch) almost disappear on that of silver colloid.⁶⁴ It shows that the primary binding mode of DDTC with silver nanoparticles is the bidentate configuration. The redistribution of the free electron pair in N atom and formation of the C=N⁺ bonding occur through modification,⁶⁵ brings about the positive DDTC modified silver nanoparticles.

Fig. 2 (a) XPS spectrum of the Ag (3d) scan of the silver colloid before (red) and after (black) modification with DDTC. (b) Raman spectrum of DDTC powder (red) and DDTC on the silver colloid (black).

Comparing to surfaces modified with primary or secondary amines, one advantage of DDTC modified silver colloid is that it is expected to carry a positive surface charge over a broad pH range for it does not rely on the solution through addition of sufficiently acidic to protonate their basic nitrogen atom.⁶⁶ As shown in Fig. S4,† their SERS spectra are identical over the pH range 2 to 12. This is vital for it means that they can be used at neutral and alkaline pH. The good surface-enhanced Raman (SERS) activity of the silver colloid can be kept in at least 5 days.

3.3. Detection of inorganic explosive anions

3.3.1. Qualitative detection of inorganic explosive anions. The potential application of positively charged DDTC-modified silver colloid for detection of inorganic explosive anions was further demonstrated. DDTC-modified silver colloid dry on the tin foil to form dense silver nanoparticles aggregates for detection of inorganic explosive anions, as shown by scanning electron microscopy (see the Fig. S5†). The high SERS signal of the aggregated DDTC modified silver nanoparticles was obtained compared with the non-aggregated DDTC silver nanoparticles as shown in Fig. S6.†^67–69 To confirm the amount of the DDTC required, the 1 mM perchlorate was detected by various volume ratios of DDTC modified silver colloid. Fig. S6† shows that the SERS intensity of the peak at 930 cm⁻¹ reached the maximum when the volume ratio of 10⁻⁴ M DDTC solution and the silver colloid is 0.35/10. The SERS spectra of the explosive anions on the DDTC-modified silver colloid are shown in Fig. 3. Compared to the Raman peak of perchlorate powder at 952 cm⁻¹ (Fig. S8†), the characteristic Raman peak of perchlorate at 930 cm⁻¹ (Fig S8a†) showed a blue shift, which may be attributed to the interaction or sorption of ClO₄⁻ with positively charged silver colloid.⁷⁰ The nitrate and PA had similar qualitative results (Fig. S8b and c†). In the absence of DDTC, no Raman signals of explosives anions could be detected by the native silver colloid, as shown in Fig. S9.† These results further illustrate the important role of the positively charged DDTC modified silver nanoparticles in detection of the explosives anions, as the receptor to bind explosives anions through the electrostatic interaction.

Fig. 3 SERS spectrum of 0.1 mM perchlorate (black), 0.1 mM nitrate (red), 0.1 mM PA (blue) extracted by the DDTC-modified silver colloid.

To evaluate the sensitivity for detecting the explosives anions of DDTC modified silver colloid, the enhancement factor (EF) also calculated as listed in the Table S2.† In addition, uniformity is an important parameter evaluating the reliability of SERS detection.²⁸ The SERS spectra of 20 points randomly selected from a DDTC-modified silver colloid probed with 1 mM perchlorate was recorded in Fig. S10a.† The intensity of the perchlorate peak at 930 cm⁻¹ had an RSD of 11.4%. The pronounced Raman peak of DDTC at 1273 cm⁻¹ assigned to the δ(HCH)(CH₃)(CH₂) was selected as an internal standard peak. The normalized intensity of the perchlorate peak at 930 cm⁻¹ had an RSD of 5.45% as shown in Fig. S10b.† These results represent the positively charged silver colloid as the substrate has excellent uniformity across the entire area. Besides, the reproducibility was evaluated. SERS measurements of 0.1 mM perchlorate in five different samples were performed and the SERS spectra were obtained (Fig. S11†). The relative standard deviation was 6.9% (RSD of Raman peak of perchlorate at 930 cm⁻¹). The consistency of the signal intensity indicated high reproducibility of the measurements.

3.3.2. Quantitative detection of inorganic explosive anions. To assess the sensor sensitivity to explosives anions, DDTC-modified silver colloid was first added into an ethanol solution containing anions at various concentrations. The Raman signals of explosives anions were recorded using a portable Raman system after the solution air-drying. Fig. 4 shows the SERS spectra of explosives anions with increasing concentrations on DDTC-modified silver colloid. The characteristic Raman bands of explosives anions were quite sensitive to its concentrations and the intensity increases gradually with the increasing of the analyte concentration. Quantitative SERS analysis of explosives anions was also carried out. The characteristic peaks of perchlorate at 930 cm⁻¹ and nitrate at 1048 cm⁻¹ were selected as the function of the explosive anion concentration with the peak of DDTC at 1273 cm⁻¹ as the internal peak. The calibration curve for explosives anions was plotted based on the internal reference method,⁷¹ which provide a reliable way to investigate the correlation between molecular structures and their SERS activities. As seen in Fig. 4, the normalized Raman signal intensity of explosives anions increases with the increasing concentrations of the analyte. The plot of the normalized Raman intensity of perchlorate at 930 cm⁻¹ and nitrate at 1048 cm⁻¹ versus its concentration exhibits a good linear correlation respectively. Meanwhile, the SERS spectra of PA at various concentration was also be recorded, and the curve shows a good liner relationship with the selected the Raman peak at 820 cm⁻¹ as the characteristic peak for normalization (Fig. 4e and f). From the liner correlation, we can obtain the linear regression equation, the limits of detection (LOD, S/N = 3) and relative standard deviation (RSD), which are summarized in Table 1. The LOD for perchlorate, nitrate and PA were 1.19 ng cm⁻², 2.01 ng cm⁻² and 8.96 ng cm⁻², respectively. In addition, the RSD were 5.18%, 5.52% and 4.46%, respectively. These results exhibit the good quantitative capability of the current method for detection of explosives anions. The detection limits should be satisfied with the rapid screening suspicious objects.

Fig. 4 (Left) Normalized SERS spectra of three explosive anions at different concentrations (a) perchlorate (b) nitrate (c) picric acid. (Right) Calibration curve for (d) perchlorate at 930 cm⁻¹, (e) nitrate at 1048 cm⁻¹, (f) picric acid at 820 cm⁻¹. Each data point represents the average value from three SERS spectra. Error bars show the standard deviations.

Table 1 Quantitative analysis results^a

Anions	Perchlorate	Nitrate	Picric acid
a LOD = limit of detection; RSD = relative standard deviation.
Liner equation	y = 4.73 × 10^−3 + 1.84 × 10^−4x	y = 4.44 × 10^−3 + 2.73 × 10^−4x	y = 3.00 × 10^−3 + 2.34 × 10^−4x
LOD (ng cm^−2)	1.19	2.01	8.96
RSD (%)	5.18	5.52	4.46

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3.3.3. In situ detection the inorganic explosives in various matrices. We have further developed the utility of this concept to detect the simulated inorganic explosives firecrackers and black powders in various matrices. A plastic and a fabric bag were chosen as target matrices which were spiked with the dilute simulated inorganic explosives ethanol solution and subsequent drying at room temperature. The simulated inorganic explosives were obtained by mixing the NO₃⁻ and ClO₄⁻ with the sugar (NO₃⁻/sugar (4 : 1), ClO₄⁻/sugar (3 : 2)) based the composition of the black powder and firecracker.³ Meanwhile, 20 μL of concentrated DDTC modified silver colloid was added onto these target matrices and retained until completely dry. The SERS spectra of simulated inorganic explosives were collected and shown in Fig. 5. The remarkable Raman signals could be obtained by detecting the 3.521 ng cm⁻² firecrackers and 31.6 ng cm⁻² black powders on the plastic. Similarly, 6.47 ng cm⁻² firecrackers and 32.2 ng cm⁻² black powders can also be detected on the fabric bag respectively.

Fig. 5 The detection of actual samples (firecrackers and black powders) on (a and b) plastic and (c and d) a synthetic fabric bag.

4. Conclusions

It is demonstrated that sodium Diethyldithiocarbamate (DDTC), a potent copper chelating agent, can be absorbed on the normal negatively charged silver nanoparticles to generate the positively charged ones. The positively charged silver colloid kept the size and dispersity. The zeta potential of positively charged silver colloid is about 32.1 mV, comparing to −43.7 mV of citrate stabilized silver colloid. The XPS and Raman spectra revealed that the DDTC induced formation of the positively charged silver nanoparticles due to the adsorption manner of the bidentate configuration of DDTC, leading to redistribution of the free electron pair in N atom and formation of the C=N⁺ bonding binding. The DDTC as the modifier can maintain the positive zeta potentials of silver colloid even at alkaline pH, in contrast to primary or secondary amines. The positively charged silver nanoparticles colloid could be employed in situ detect the inorganic explosives anions NO₃⁻ and ClO₄⁻ on the surface with a portable Raman spectrometer through the electrostatic interaction. In addition, the DDTC could also serve as an internal standard in quantitative analysis. The concentrated DDTC-modified silver nanoparticles showed a good uniformity. The SERS intensity versus the explosives anions concentration showed a good linear relationship in the concentration range from 1 to 500 ng cm⁻². The detection limit of simulated firecrackers and simulated black powders mainly composed of ClO₄⁻ or NO₃⁻ could reach 3.521 ng cm⁻², 31.6 ng cm⁻² on the plastic, and 6.47 ng cm⁻², 32.2 ng cm⁻² on fabric bag respectively. These results showed that the common plasmonic metal nanomaterials could facilely be transformed into the positive-charged ones with DDTC surface modification, which could be contributed to the trace analysis of inorganic anion oxidizer in crime scene investigation and homeland security combining with a portable Raman spectrometer.

Acknowledgements

We thank the financial support from the National Basic Research Program of China (973 Program 013CB934301), the National Natural Science Foundation of China (NSFC21377068 21575077).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra06111g

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