Visual colorimetric detection of TNT and 2,4-DNT using as-prepared hexaazamacrocycle-capped gold nanoparticles

Abstract

The dual role of a hexaazamacrocyclic ligand L in the effective reduction of Au³⁺ to Au⁰ and stabilization of the ensuing gold nanoparticles (AuNPs) is successfully demonstrated. The molar ratio of ligand L to Au³⁺ is found to be very important for the effective stabilization of the AuNPs. The as-prepared amine-capped AuNPs in the instant visual colorimetric detection of the TNT and 2,4-DNT under easy-to-adopt conditions is established. The naked eye detection of the analytes with an instant color change at concentrations as low as 476 fM is observed. Colorimetric analysis supported the detection of TNT and 2,4-DNT using the nanosensor with the LOD of 1.55 × 10⁻¹³ ± 2.02 × 10⁻¹³ M and 3.79 × 10⁻¹³ ± 3.65 × 10⁻¹³ M respectively.

---

Introduction

Gold nanoparticles (AuNPs) play vital roles in several research areas including the study of chemosensors,¹ biosensors,^2–4 cancer therapy,^5–7 and optoelectronics.⁸ Optical properties like SPR, SERS and luminescence are monitored to comprehend the above-mentioned applications of AuNPs. These optical properties are understandably dependent on the size as well as the shape of the particles. Various parameters such as the temperature,⁹ pH,¹⁰ molar fraction of precursor to reducing agent¹¹ and stabilizer^10a,12 are known to modulate the size and shape of the AuNPs. Thiols are traditionally used as stabilizers during the synthesis of AuNPs.^10a,12,13 However, the use of amines as stabilizers has been a recent trend. Amines could display bifunctional character in the synthesis of AuNPs by offering a dual role, i.e. as a reducing and stabilizing agent.¹⁴ Blanchard's group has studied the reduction of Au³⁺ to Au⁰ using a variety of amines as a reducing agent. It is found that amines with a reduction potential within the range of oxidation potential of Au⁰ to Au⁺ and the reduction potential of Au³⁺ to Au⁰ are suitable for the required efficient reduction of Au³⁺ in the formation of AuNPs.¹⁵ They have also reported the use of poly(allylamine) hydrochloride as a reducing agent in the size-controlled formation of AuNPs. The size control could be achieved by varying the ratio of PAH : HAuCl₄, wherein each of the nitrogen centres functions as a one-electron donor in the reduction of Au³⁺ to Au⁰.^12b

Detection of the commonly used secondary explosives like TNT and 2,4-DNT when present in a given sample should be possible by employing suitable amine-stabilized AuNPs. Nitro derivatives are known to act as an acceptor and form Meisenheimer complexes when combined with donors such as amines.¹⁶ Thus, amine-modified AuNPs are potential candidates to study their applications as a sensor for the detection of TNT and 2,4-DNT. The interactions of donor amine groups (present in amine-functionalized AuNPs) with the acceptor nitro-compounds (that are employed as analytes) promote the aggregation of NPs. The concomitant aggregation of amine-functionalized AuNPs when exposed to nitro derivatives is actually perceived by various techniques like colorimetry,^1a fluorimetry,^1b,17 SERS,¹⁸ DLS,¹⁹ electrochemical methods²⁰ and miscellaneous methods.²¹ Among these the colorimetric technique is considered as the least expensive and most convenient to execute and hence is suitable for application purposes. For instance, aggregation of cysteamine-modified AuNPs in the presence of TNT is reported whereupon TNT is detected colorimetrically.^1a

As per previous reports, a variety of AuNPs are prepared via citrate reduction in the first step and then functionalized with amines in the next step for their use in the detection of nitro derivatives.^{1,18a,b,19,20b,22} In the first step, AuNPs are prepared by reducing Au³⁺ using an external reducing agent and an appropriate stabilizer. The AuNPs so prepared are further modified with suitable amines in a subsequent step and then used as sensors of compatible nitro derivatives. It should be possible to prepare AuNPs by using the chosen amines as stabilizers so that no post-modification is warranted. In case the chosen amine is also capable of reducing the metal ion, the use of an external reducing agent could be avoided, and thus possible contaminations are curtailed. Keeping in mind the above detail, we have used a hexaazamacrocyclic ligand L that displays a dual role of reducing and stabilizing agent during the synthesis of a new class of amine-stabilized AuNPs. The synthesis of the AuNPs could be accomplished under mild reaction conditions. Since non-functionalized amine-capped AuNPs synthesized in a one-pot reaction are utilized as a sensor, it not only reduces the number of steps in the nanosensor synthesis, but also possible sensor contaminants such as citrate or other stabilizer and reducing agents usually used prior to the functionalization step could be avoided. This new system has been successfully used in the present work for the naked-eye and colorimetric detection of TNT and 2,4-DNT under neat and easy-to-adopt conditions, even at the femtomolar level.

Experimental

Materials and characterization techniques

The hexaazamacrocyclic ligand L was prepared following a literature method.²³ The metal salt HAuCl₄·3H₂O (99.98%) was purchased from Aldrich. UV-visible spectral studies were performed using a JASCO V-650 instrument. TEM images were recorded using a Philips CM12 TEM with (EDS) detector. Powder XRD patterns were obtained using a Bruker D8 Advance instrument.

Procedure

Synthesis of hexaazamacrocycle-capped AuNPs. A 5 mM stock solution of Au³⁺ was prepared by dissolving HAuCl₄·3H₂O (0.0985 g, 0.05 mmol) in 50 mL of Millipore water. A 10 mM stock solution of the hexaazamacrocyclic ligand was prepared by dissolving L (0.2050 g, 0.10 mmol) in 50 mL of methanol. The syntheses of the AuNPs were attempted at various molar ratios of ligand L to Au³⁺, i.e. 0.25 : 1; 0.5 : 1; 1 : 1; 2 : 1 and 3 : 1. In all experiments the final concentration of Au was kept at 1 mM. In a typical synthesis an aliquot of 2 mL of 5 mM aqueous solution of Au³⁺ was added to the required volume of the solution of L in such a manner that the total volume and ratios of H₂O : MeOH were maintained as 5 mL and 1 : 1, respectively. The reaction mixtures were allowed to stand at room temperature for 2 weeks to ensure complete growth. Among the above trials, 2 or 3 equivalents of the ligand L appears to be required for the complete reduction and stabilization of the AuNPs. When 1 equivalent of ligand L was used, although complete reduction was observed the NPs formed were deposited. We found that a minimum of 2 equivalents of ligand L is required for stabilization of the AuNPs.

The preparation of AuNPs was also attempted at a low concentration of Au (e.g. 0.1 mM) by retaining the optimized ratio of ligand to metal, i.e. 2 : 1. At this low concentration of Au, which is lower than the above considered 1 mM, the reaction mixture was heated at 50–60 °C for 16 h to ensure complete reduction. Further, the mixture was allowed to stand for 2 weeks to ensure complete growth.

Application of as-prepared AuNPs in visual and colorimetric detection of TNT and 2,4-DNT. Stock solutions of the nitro-compounds were prepared at 1 mM by dissolving nitrobenzene (1 μL), p-nitrotoluene (0.0014 g), o-nitrophenol (0.0014 g), p-nitrophenol (0.0014 g), and picric acid (0.0023 g) in 10 mL of deionised water whereas TNT (0.0023 g), 2,4-DNT (0.0018 g), 3,4-DNT (0.0018 g) were dissolved in 10 mL of MeOH in separate standard flasks. Further dilutions of the 1 mM solutions of TNT and 2,4-DNT were carried out by addition of the required amount of MeOH to prepare solutions up to 1 pM in concentration. To 3 mL of AuNPs (1 mM in Au), 0.1 mL of a 1 mM solution of the chosen nitro-compound was added whereupon the concentration of the nitro-compound in the final solution became 32 μM. Samples were observed for visual colour change. UV-vis spectra were recorded for each sample.

Similar experiments were also conducted using AuNPs prepared at a lower concentration of Au. To 3 mL of AuNPs (0.1 mM in Au), 0.1 mL of the required concentration of TNT and 2,4-DNT was added whereupon the concentration of the nitro-compounds in the final solution remained in the range of 47.6 μM to 476 fM. Samples were monitored for visual colour change. UV-vis spectra were recorded for each sample.

To 3 mL of the as-prepared AuNPs (0.1 mM Au), 0.1 mL of 1 mM aqueous solutions of LiCl, NaCl, KCl, MgCl₂, CaCl₂, MnCl₂, CuCl₂, FeCl₂, NiCl₂ and ZnCl₂ were added separately in batches. UV-vis spectra were recorded for each sample.

Results and discussion

Recently, we reported the use of a hexaazamacrocyclic ligand as the sensing agent of Ag⁺via silver nanoparticle formation.^14j In continuation of our efforts, herein we synthesized gold nanoparticles (AuNPs) using the hexaazamacrocycle, L under mild conditions and utilized the as-prepared AuNPs in the visual colorimetric detection of TNT and 2,4-DNT selectively up to the femtomolar level.

In the synthesis of the AuNPs, the molar ratio of L to Au³⁺ is found to influence the feasibilities of reduction of the Au³⁺ and also stabilization of the ensuing NPs. During the synthesis of the AuNPs, the molar ratio of L to Au³⁺ was varied by increasing the concentration of L with respect to Au³⁺ ion in the samples. In the case of molar ratio values less than 2 it was observed that the solutions were turbid with a yellowish colouration. This suggests incomplete reduction of Au³⁺ to Au⁺ and Au⁰ due to shortage of the ligand which resulted in the yellow colouration. The stabilization of the AuNPs is also hampered due to the shortage of the ligand, and nucleated particles began to be deposited at the walls of the vial. When the molar ratio of L to the metal salt in a sample is sufficiently high, such as 2 : 1 or higher, the solutions were supersaturated with the Au⁰ nucleated particles leading to the formation of stable AuNPs. Thus the formation of stable AuNPs is facilitated in such situations and the colour changed from yellow to yellowish orange instantly and then slowly it turned to red over time. The colour change from yellow to red is attributed to the formation of spherical AuNPs.

UV-visible spectra were recorded for each sample with different molar ratios as shown in Fig. 1a. The spectra showed SPR peaks at 523 nm for the samples obtained from 2L : 1Au³⁺ and 3L : 1Au³⁺ solutions. The SPR peaks confirm the formation of AuNPs. The as-prepared AuNPs are found to have excellent stability in the solution state for more than a year. At lower molar ratios, no peak was observed at the 525 nm region; however, at 400 nm a broad peak was seen probably due to incomplete reduction, the small size of AuNPs or formation of a coordination complex with Au⁺. The influence of the increase of temperature for the sample at a molar ratio below 2 for L : Au³⁺ was probed to check whether or not it is possible to drive the reactions favorably, but there was no improvement. This study suggested that a 2 : 1 molar ratio is required for the effective preparation and stabilization of the AuNPs.

Fig. 1 (a) UV-visible spectra of AuNPs prepared using different molar ratio of ligand L to Au in the range of 0.25–3, where the concentration of Au was kept constant as 1 mM; (b) TEM image of L-stabilized AuNPs prepared by using a 2 : 1 molar ratio of ligand to metal (1 mM of Au³⁺ was taken) after aging the AuNPs for 2 weeks.

The attempts to prepare AuNPs in the range of 1–2 for the molar ratio of L : Au³⁺, i.e. 1, 1.11, 1.25, 1.43, 1.67 and 2, resulted in the successful stabilization of AuNPs only when the ratio is 2. A colour change was observed from orange to red after 1 day for the case of the molar ratio 2 : 1 and as the time evolved the colour of the solution became intense. UV-vis spectra were recorded for each sample after 3 days, and an intense peak at 523 nm was observed for samples containing L : Au³⁺ of 2 : 1. The AuNPs prepared in less than 2 : 1 molar ratios formed flocs and the UV-vis spectra showed a broad peak in the longer wavelength region due to the unstable particles leading to flocculation/aggregation as shown in Fig. 2a.

Fig. 2 (a) UV-visible spectra of AuNPs prepared using different molar ratios of ligand L to Au, in the range of 1–2; (b) TEM image of flocculated AuNPs prepared by using a 1 : 1 molar ratio of ligand to metal (1 mM of Au³⁺ was taken) and allowed to stand only for 8 h.

The TEM image of L-stabilized AuNPs prepared at the 2 : 1 ratio was recorded after aging the sample for 2 weeks and is shown in Fig. 1b. The average size of the AuNPs was found to be 13.06 ± 3.46 nm. However, the TEM image taken for the attempted preparation of AuNPs at 1 : 1 molar ratio of L : Au³⁺ showed flocculated/aggregated AuNPs as shown in Fig. 2b.

Powder XRD of the well stabilized AuNPs prepared at the 2 : 1 ratio showed face-centred cubic lattice planes of Au with 2θ values at 38.219°, 44.396°, 64.666°, 77.665° and 81.882° as shown in Fig. 3 corresponding to the (111), (200), (220), (311) and (222) planes. The 2θ values are comparable with the data reported in JCPDS 89-3697.

Fig. 3 Powder XRD patterns of AuNPs prepared by using the 2 : 1 molar ratio of ligand to metal (1 mM of Au³⁺ was taken) after aging the AuNPs for 2 weeks.

The colour of the AuNPs changed spontaneously from red to blue when combined with TNT or 2,4-DNT. In addition to the SPR peak of the AuNPs another broad peak was observed at around 600 nm in both cases as shown in Fig. 4. The presence of the peak around 600 nm is a clear indication of the aggregation of AuNPs. This aggregation is attributed to Meisenheimer complex formation between the amine groups present at the surface of the AuNPs and TNT or 2,4-DNT that is used as the analyte.^18a Picric acid and 3,4-DNT are analogues of TNT and 2,4-DNT, respectively. However, both of these nitro-compounds did not show any immediate colour change where the SPR of AuNPs also remained unchanged as shown in Fig. 4. In the case of picric acid, when sufficient time is provided in the order of 10–12 h then a visible colour change from red to purple was observed. The reason for this slow reactivity of picric acid may be attributed to the less electron-deficient nature of its π system compared to that of TNT, and also it can form salts with the amine – this salt formation can be evidenced from the peak observed in the 360 nm region. However, in the case of 3,4-DNT the colour of AuNPs remained unchanged even after a long reaction time. It is a well-known fact that 3,4-DNT is less electron-deficient when compared with 2,4-DNT. This could account for the behaviour of 3,4-DNT as a poor acceptor in detection.

Fig. 4 UV-vis spectra of (1 mM) AuNPs before and after addition of picric acid, 3,4-DNT, 2,4-DNT and TNT. The spectra were recorded immediately.

To investigate the selectivity for nitro-compounds, we screened some other derivatives such as nitrobenzene, nitrotoluene, o-nitrophenol, and p-nitrophenol for the detection. While TNT and 2,4-DNT were detected spontaneously, picric acid required a longer time for the detection. The other compounds were not detected as concluded from the UV-vis studies, see Fig. S1, ESI.†

To further support the argument, TEM characterizations were made for the sample in the absence and presence of nitro-compounds. In the case of TNT, or 2,4-DNT, TEM samples were prepared immediately after the addition of TNT and 2,4-DNT to the AuNPs and the solvent was removed by touching the grid gently with tissue paper. In the case of the sample containing picric acid the TEM sample was prepared 1 day after the addition of picric acid to the as-prepared AuNPs.

The TEM image of the as-prepared AuNPs (prepared from 0.1 mM HAuCl₄) showed complete dispersion of AuNP particles, as shown in Fig. 5a. The average size of the AuNPs was found to be 11.47 ± 2.62 nm. The TEM image of the AuNPs after the addition of TNT/2,4-DNT showed the aggregation of particles with no significant increase in the sizes of the particles. The images of samples containing 2,4-DNT, TNT and picric acid are shown in Fig. 5b–d. The results strongly suggest that the colour change observed was only due to aggregation and not because of any particle growth that can occur by coalescence of unprotected particles. Hence, the amine ligand is found to be proficient for capping the AuNPs strongly, even after the addition of TNT or 2,4-DNT. The aggregation is attributed to the formation of Meisenheimer complexes.

Fig. 5 TEM images of AuNPs (0.1 mM in Au): (a) as-prepared, avg. size of the particles = 11.47 ± 2.62 nm; (b) immediately after addition of 0.1 mL of 0.1 mM 2,4-DNT, avg. size of the particles = 11.29 ± 1.88 nm; (c) immediately after addition of 0.1 mL of 0.1 mM TNT, avg. size of the particles = 11.54 ± 2.45 nm; and (d) 1 day after of addition with 0.1 mL of 1 mM picric acid, avg. size of the particles = 9.54 ± 2.22 nm.

The possibility of interference from the metal ions of alkali, alkaline and transition series was probed by adding their salts to the AuNPs. The samples of LiCl, NaCl, KCl, MgCl₂, CaCl₂, MnCl₂, CuCl₂, FeCl₂, NiCl₂ and ZnCl₂ did not show any changes in the peak positions in the UV-vis spectra as shown in Fig. 6, where the concentration of Au in the samples was 0.1 mM and with the final concentration of the analyte metal salts at 32 μM.

Fig. 6 UV-vis spectra of the AuNPs in the presence of metal ions.

To further assess the sensitivity of the simple colorimetric assay of the nitro-compounds, a batch of experiments was carried out at various lower concentrations of the nitro-compounds. Wherein, after the addition of various concentrations of TNT or 2,4-DNT, ranging from 476 fM to 47.6 μM, to the as-prepared AuNPs, an immediate colour changed from red to blue was observed. The observed change in the colour of the samples is due to aggregation of the AuNPs. A photograph of the AuNPs with and without TNT is shown in Fig. 7. The analytes could be detected by amine-capped AuNPs with a visual color change instantly, even to a concentration as low as the 476 fM level. The UV-vis spectra of the samples with the increase in the concentration of the analyte are found to show a red shift with broadening of the SPR peak. The UV-vis study for the analyte TNT is shown in Fig. 8a whereas that of 2,4-DNT is shown in Fig. S2a, ESI.† The detection limit of the analyte was calculated from the plot of log[TNT] or [2,4-DNT] vs. A_final/A_initial respectively. The average detection limit values of three batches of experiments is found to be 1.55 × 10⁻¹³ ± 2.02 × 10⁻¹³ M for TNT and 3.79 × 10⁻¹³ ± 3.65 × 10⁻¹³ M for 2,4-DNT. The plot related to the detection limit study for TNT is shown in Fig. 8b whereas that of 2,4-DNT is given in Fig. S2b, ESI.†

Fig. 7 Photograph showing the color change of the AuNPs when treated with TNT. From right to left: AuNPs in the absence of TNT and gradual increase of TNT concentration by adding 0.1 mL of 10 pM, 0.1 nM, 1 nM, 10 nM, 0.1 μM, 1 μM, 10 μM, 0.1 mM, 0.5 mM and 1 mM TNT from the stock solutions.

Fig. 8 (a) UV-vis spectra of AuNPs (0.1 mM in metal) before and after the addition of different concentrations of TNT, where the concentration of the nitro-compound in the final solutions ranges from 47.6 μM to 476 fM. (b) Plot of A_final/A_initial against log[TNT] for TNT assay.

As per previous reports, a variety of AuNPs are prepared via citrate reduction or so in the first step and then functionalized with amines in the next step for their use in the detection of nitro derivatives.^{18a,b,19,20b,22} The LOD for TNT and 2,4-DNT in the present study is as low as 155 fM and 359 fM respectively, using AuNPs by a simple colorimetric method. Reported methods and LOD values for TNT and 2,4-DNT detection using nanoparticles are collected in Table S1 in the ESI†. The comparison shows that our method is one of the best reported so far. There is one report with an LOD at the zeptomolar level for the detection of TNT.^17e However, a fluorescence technique is required for the detection and the synthesis of the sensor involves a time-consuming multi-step process. Further, under colorimetric conditions the LOD is found to be higher than 44 μM. To the best of our knowledge all the reports using NPs as the sensor for TNT involve a functionalization/modification step of the NPs, mostly with amines. Expensive equipment is employed for the detection of the analytes in the reported methods. The novelty of the present work is that the as-prepared AuNPs were suitable for the detection of compatible nitro derivatives without any further modification.

Conclusions

In summary, AuNPs were synthesized using the hexaazamacrocycle L, and the as-prepared AuNPs were successfully utilized in the spontaneous visual and colorimetric detection of TNT and 2,4-DNT at the femtomolar level.

Acknowledgements

D.K.C. thanks CSIR, India for financial support.

References

1. (a) Y. Jiang, H. Zhao, N. Zhu, Y. Lin, P. Yu and L. Mao, Angew. Chem., Int. Ed., 2008, 47, 8601; (b) N. Tu and L. Wang, Chem. Commun., 2013, 49, 6319.
2. C. Guo, P. Boullanger, L. Jiang and T. Liu, Biosens. Bioelectron., 2007, 22, 1830.
3. X. Mao, Y. Ma, A. Zhang, L. Zhang, L. Zeng and G. Liu, Anal. Chem., 2009, 81, 1660.
4. D. Zhang, O. Neumann, H. Wang, V. M. Yuwono, A. Barhoumi, M. Perham, J. D. Hartgerink, P. Wittung-Stafshede and N. J. Halas, Nano Lett., 2009, 9, 666.
5. A. Ambrosi, F. Airò and A. Merkoçi, Anal. Chem., 2009, 82, 1151.
6. B. Kang, M. A. Mackey and M. A. El-Sayed, J. Am. Chem. Soc., 2010, 132, 1517.
7. S. Lee, S. Kim, J. Choo, S. Y. Shin, Y. H. Lee, H. Y. Choi, S. Ha, K. Kang and C. H. Oh, Anal. Chem., 2007, 79, 916.
8. M. E. van der Boom, Angew. Chem., Int. Ed., 2002, 41, 3363.
9. G. Palui, S. Ray and A. Banerjee, J. Mater. Chem., 2009, 19, 3457.
10. (a) H. Xia, S. Bai, J. r. Hartmann and D. Wang, Langmuir, 2009, 26, 3585; (b) J. D. S. dos Santos, R. A. Alvarez-Puebla, J. O. N. Oliveira and R. F. Aroca, J. Mater. Chem., 2005, 15, 3045.
11. J. D. S. Newman and G. J. Blanchard, J. Nanopart. Res., 2007, 9, 861.
12. (a) T. Selvam and K.-M. Chi, J. Nanopart. Res., 2011, 13, 1769; (b) B. Kim, S. L. Tripp and A. Wei, J. Am. Chem. Soc., 2001, 123, 7955.
13. J. Song, D. Kim and D. Lee, Langmuir, 2011, 27, 13854.
14. (a) M. Aslam, L. Fu, M. Su, K. Vijayamohanan and V. P. Dravid, J. Mater. Chem., 2004, 14, 1795; (b) C. Graf and A. van Blaaderen, Langmuir, 2001, 18, 524; (c) A. Kumar, S. Mandal, R. Pasricha, A. B. Mandale and M. Sastry, Langmuir, 2003, 19, 6277; (d) D. V. Leff, L. Brandt and J. R. Heath, Langmuir, 1996, 12, 4723; (e) H. Nakao, H. Shiigi, Y. Yamamoto, S. Tokonami, T. Nagaoka, S. Sugiyama and T. Ohtani, Nano Lett., 2003, 3, 1391; (f) P. R. Selvakannan, S. Mandal, S. Phadtare, R. Pasricha and M. Sastry, Langmuir, 2003, 19, 3545; (g) K. G. Thomas and P. V. Kamat, J. Am. Chem. Soc., 2000, 122, 2655; (h) K. G. Thomas, J. Zajicek and P. V. Kamat, Langmuir, 2002, 18, 3722; (i) J. Athilakshmi and D. Chand, J. Chem. Sci., 2011, 123, 875; (j) J. Athilakshmi and D. K. Chand, Tetrahedron Lett., 2010, 51, 6760.
15. J. D. S. Newman and G. J. Blanchard, Langmuir, 2006, 22, 5882.
16. F. Terrier, Chem. Rev., 1982, 82, 77.
17. (a) J. Geng, P. Liu, B. Liu, G. Guan, Z. Zhang and M.-Y. Han, Chem.–Eur. J., 2010, 16, 3720; (b) L. Feng, H. Li, Y. Qu and C. Lu, Chem. Commun., 2012, 48, 4633; (c) D. Gao, Z. Wang, B. Liu, L. Ni, M. Wu and Z. Zhang, Anal. Chem., 2008, 80, 8545; (d) R. Tu, B. Liu, Z. Wang, D. Gao, F. Wang, Q. Fang and Z. Zhang, Anal. Chem., 2008, 80, 3458; (e) A. Mathew, P. R. Sajanlal and T. Pradeep, Angew. Chem., Int. Ed., 2012, 51, 9596.
18. (a) S. S. R. Dasary, A. K. Singh, D. Senapati, H. Yu and P. C. Ray, J. Am. Chem. Soc., 2009, 131, 13806; (b) T. Demeritte, R. Kanchanapally, Z. Fan, A. K. Singh, D. Senapati, M. Dubey, E. Zakar and P. C. Ray, Analyst, 2012, 137, 5041; (c) M. Liu and W. Chen, Biosens. Bioelectron., 2013, 46, 68; (d) X. Liu, L. Zhao, H. Shen, H. Xu and L. Lu, Talanta, 2011, 83, 1023.
19. (a) S. S. R. Dasary, D. Senapati, A. K. Singh, Y. Anjaneyulu, H. Yu and P. C. Ray, ACS Appl. Mater. Interfaces, 2010, 2, 3455; (b) D. Lin, H. Liu, K. Qian, X. Zhou, L. Yang and J. Liu, Anal. Chim. Acta, 2012, 744, 92.
20. (a) S. Hrapovic, E. Majid, Y. Liu, K. Male and J. H. T. Luong, Anal. Chem., 2006, 78, 5504; (b) M. Riskin, R. Tel-Vered, T. Bourenko, E. Granot and I. Willner, J. Am. Chem. Soc., 2008, 130, 9726.
21. (a) K. Qian, H. Liu, L. Yang and J. Liu, Analyst, 2012, 137, 4644; (b) T. Kawaguchi, D. R. Shankaran, S. J. Kim, K. Matsumoto, K. Toko and N. Miura, Sens. Actuators, B, 2008, 133, 467; (c) S. S. R. Dasary, A. K. Singh, D. Senapati, H. Yu, M. Dubey, P. Amirtharaj and P. C. Ray, IEEE Trans. Nanotechnol., 2011, 10, 1083.
22. X. Zhou, H. Liu, L. Yang and J. Liu, Analyst, 2013, 138, 1858.
23. D. Chen and A. E. Martell, Tetrahedron, 1991, 47, 6895.

---

Footnote

† Electronic supplementary information (ESI) available: UV-vis spectra of AuNPs containing nitro-compounds, and also that of sensitivity study of 2,4-DNT. A table describing reported methods and LOD for TNT and 2,4-DNT detection using nanoparticles. See DOI: 10.1039/c3ay41824c

---