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 Table of Contents  
ORIGINAL ARTICLE
Year : 2016  |  Volume : 2  |  Issue : 4  |  Page : 195-202

A Gold Nanoparticle-enhanced Surface Plasmon Resonance Aptasensor for the Detection of 2,4,6-trinitrotoluene


1 Collaborative Innovation Center of Judicial Civilization; People's Public Security University of China, Beijing 100038, China
2 Collaborative Innovation Center of Judicial Civilization; Key Laboratory of Evidence Science, China University of Political Science and Law, Ministry of Education, Beijing 100088, China
3 People's Public Security University of China, Beijing 100038, China

Date of Web Publication9-Jan-2017

Correspondence Address:
Hongxia Hao
Collaborative Innovation Center of Judicial Civilization, Beijing 100088; Key Laboratory of Evidence Science, China University of Political Science and Law, Ministry of Education, Beijing 100088
China
Ruiqin Yang
People's Public Security University of China, Beijing 100038
China
Login to access the Email id

Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2349-5014.197934

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  Abstract 

A gold nanoparticle-enhanced surface plasmon resonance (SPR) aptasensor was developed for high-specificity and high-sensitivity detection of 2,4,6-trinitrotoluene (TNT). Self-assembly film-forming technology was used to modify the gold surface of the sensor chip with 2,4,6-trinitrophenyl glycine, a TNT analogue, using polyethylene glycol to which the thiol group and carboxyl group are attached. Aptamer-gold nanoparticle complexes were formed through Au-S bonding. To detect TNT, the samples were incubated with the aptamer-gold nanoparticle complexes, and the solution competition method was applied through the SPR aptasensor. The results showed that the SPR aptasensor achieved fast, real-time detection of TNT. This gold nanoparticle-enhanced SPR aptasensor is suitable for TNT detection in the field of public safety and environmental monitoring.

Keywords: Aptamer, surface plasmon resonance, TNT


How to cite this article:
Tan J, Hao B, Wang C, Ren Y, Hao H, Yang R. A Gold Nanoparticle-enhanced Surface Plasmon Resonance Aptasensor for the Detection of 2,4,6-trinitrotoluene. J Forensic Sci Med 2016;2:195-202

How to cite this URL:
Tan J, Hao B, Wang C, Ren Y, Hao H, Yang R. A Gold Nanoparticle-enhanced Surface Plasmon Resonance Aptasensor for the Detection of 2,4,6-trinitrotoluene. J Forensic Sci Med [serial online] 2016 [cited 2019 Aug 25];2:195-202. Available from: http://www.jfsmonline.com/text.asp?2016/2/4/195/197934


  Introduction Top


Explosives are used in organized crimes and terrorist attacks to induce panic. Explosions cause adverse social impact and threaten life, health, and property. 2, 4, 6-Trinitrotoluene (TNT) is commonly used in explosives because of its strong explosive force and excellent comprehensive performance. When TNT remains in the environment after an explosion, it is highly toxic to both the environment and human health. Fast, effective, and accurate detection of TNT (and particularly trace TNT) is essential to address these harmful threats to the public and environmental safety.

TNT detection through conventional ion mobility spectrometry and gas chromatography-mass spectrometry requires the use of large-scale, expensive equipment, complex preprocessing procedures, and specialized expertise.[1],[2],[3] These methods cannot meet the urgent demand for fast field detection. In recent years, highly sensitive, less expensive, and portable sensors have been developed. Commonly used sensors for detecting TNT include microelectromechanical sensors, surface acoustic wave sensors, fluorescent molecular sensors, electrochemical sensors, molecular imprinted polymer sensors, and surface plasmon resonance (SPR) sensors. SPR sensors, which integrate the advantages of all other types of sensors, can dynamically and immediately detect target analytes in turbid or opaque samples, without the need for labeling the samples.[4],[5],[6] These features make SPR sensors widely applicable for TNT detection. Shankaran et al.[7],[8],[9] modified SPR sensors through physical adsorption of proteins, which was then applied in TNT detection based on competitive immunoassay. This method, however, has a low detection limit. Later Kawaguchi et al.[10],[11] and Singh et al.[12] described SPR sensors modified with polyethylene glycol (PEG) or highly branched polyamidoamine dendrimers, which effectively increased the loading capacity and detection efficiency of the SPR sensor chip. Most SPR sensor-based TNT detection methods function under the principle of immunoreactivity, which involves complex and expensive procedures, and the prepared antibodies must be stored and transported under rigorous conditions. These strict requirements have restricted the applications of SPR sensors, and thus a simple, effective, and less expensive SPR sensor is needed to detect trace TNT.

Aptamers are oligonucleotide or peptide molecules with a specific three-dimensional structure and bind to a specific target molecule through coiling and folding. Aptamers have many advantages including wide availability, in vitro screening capability, nonimmunogenicity, structural stability, high specificity, and high affinity in molecular recognition that rival the capabilities of antibodies.[13],[14],[15],[16],[17],[18] Compared to DNA aptamers, peptide aptamers typically carry a thiol group, which is easily modified. This feature has been utilized to form complexes with gold nanoparticles through Au-S bonding. In Au-S bonding, the specificity and affinity of the aptamers to the target molecules are combined with the optical characteristics of the gold nanoparticles.

In this study, an SPR sensor was modified using PEG and 2, 4, 6-trinitrophenyl glycine (TNP-Gly). The anti-TNT peptide aptamer-gold nanoparticle complexes were self-assembled, and a gold nanoparticle-enhanced SPR sensor was developed to detect TNT. The SPR sensor with thiol-PEG and thiol-PEG mixed with TNP-Gly was modified to confer the sensor with antipollution properties. Ligands, which were used for sensing, were prepared by forming anti-TNT peptide aptamer-gold nanoparticle complexes through Au-S bonding, and indirect competition was initiated for molecule recognition. The flowchart of TNT detection using a gold nanoparticle-enhanced SPR aptamer is shown in [Figure 1]. Electric field coupling between the gold nanoparticles and the chip's gold film magnified the signals. This study analyzed the preparation of gold nanoparticles with different diameters, formation of aptamer-gold nanoparticle complexes, sensing principle, and detection conditions of diameter of the prepared gold nanoparticles.
Figure 1: Flowchart of 2,4,6-trinitrotoluene detection using gold nanoparticle-enhanced surface plasmon resonance aptamer

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  Experimental Conditions Top


Materials and reagents

TNT, nitrotoluene, 2,4-dinitrotoluene, cyclotrimethylenetrinitramine, and TNP methyl nitramine were purchased from Beijing Institute of Technology (Beijing, China). TNP-Gly was purchased from Strategic BioSolutions (Newark, DE, USA). Thio-PEG and thiol-PEG-acid were purchased from Polypure (Oslo, Norway). 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC), n-hydroxysuccinimide (NHS), and bovine serum albumin (BSA) were purchased from Sangon Biotech (Shanghai, China). A specific peptide aptamer targeting TNT was used and purified through high-performance liquid chromatography (GL Biochem, Shanghai, China). Its sequence was: (N-terminal) Trp-His-Trp-Gln-Arg-Pro-Leu-Met-Pro-Val-Ser-Ile-Lys-Cys (C-terminal). Sodium dodecyl sulfate and triethylamine were purchased from ANPEL Laboratory Technologies (Shanghai, China). Dimethylformamide (DMF), ethylenediamine (EDA), chloroauric acid hydrate, and trisodium citrate dihydrate were purchased from Sinopharm Group Co., Ltd. (Shanghai, China). Ultrapure water was obtained using the Millipore filtration system (18.2 MΩ, Millipore, Billerica, MA, USA). River water samples were collected from three sites on the Qing River in Haidian District, Beijing.

Equipment

For the SPR detection system, the Kretschmann system was used to design the SPR instrument. 50 Conc UV-vis spectrophotometer (Varian Medical Systems, Palo Alto, CA, USA), Tecnai G 2 F30 field emission transmission electron microscope (TEM; FEI, Hillsboro, OR, USA), PDC-32G-2 basic plasma cleaner (Mycro Technologies Co., Ltd., Hong Kong, China), HY-5A cyclotron oscillator (Jonghua Instrument Manufacture Co., Ltd., Jiangsu, China), DF-101S Heat-gathering Magnetic Heating Stirrer (Greatwall Scientific Industrial and Trade, Zhengzhou, China), S210 SevenCompact pH meter (Mettler-Toledo, Columbus, OH, USA), and MB100 Thermostatic Microplate Shaker (Allsheng Instruments Co., Ltd., Hangzhou, China) were used. All glass equipment was soaked in chromosulfuric acid (0.05 g/mL, in concentrated sulfuric acid) and newly prepared nitrohydrochloric acid (HCl: HNO3 = 3:1, v/v), and then thoroughly washed with ultrapure water before use.

Modification of the surface plasmon resonance sensor

A cleaned chip was soaked in a mixture of HO-PEG7-CH2 CH2-SH (3 mM) and HOOCCH2 CH2-PEG8-SH (0.3 mM) at a molar ratio of 10:1 in ethanol. After sealing, the chip was placed into a cyclone oscillator, and reaction was initiated at a rotation speed of 45 r/min at room temperature for 16 h. A self-assembled layer was formed on the chip surface. The chip was repeatedly washed with ethanol and ultrasonically treated for 1 min. The reaction was conducted by placing the chip into the mixture of EDC (0.4 mM) and NHS (0.1 mM) with a volume ratio of 1:1 at 17°C for 1 h at a rotation speed of 45 r/min. This process activated the carboxyl groups in the self-assembled layer into NHS ester. The chip was then repeatedly washed with water, ultrasonically treated for 1 min, and then placed in 0.45 mM EDA (dissolved in 0.1 M borate buffer, pH 8.5) at room temperature for 1 h with a rotation speed of 45 r/min. The EDA-modified sensor was repeatedly washed with ultrapure water, ultrasonically treated for 1 min, and dried by nitrogen gas blowing.

TNP-Gly was also activated. First, 200 µL of TNP-Gly solution (50 mM, dissolved in DMF) was loaded into the centrifuge tube and added to the mixture of 200 µL EDC (400 mM, dissolved in water) and 200 µL NHS (100 mM, dissolved in DMF). The reaction was initiated by properly mixing in 100 µL DMF at room temperature for 1 h. Finally, the pH value was adjusted to alkaline by adding triethylamine (20 M, dissolved in DMF).

The chip was placed into a clean container, and 600 µL activated TNP-Gly was added dropwise onto the surface of the EDA-modified chip to react at room temperature for 2 h. After the reaction, excess solution was removed from the corners of the chip, and the chip surface was repeatedly rinsed with a large amount of ultrapure water. After ultrasonic treatment, the chip was dried by nitrogen gas blowing for 1 min. The modified chip was placed in a container, sealed, and preserved at 4°C.

Preparation of gold nanoparticles

Gold nanoparticles were prepared using the reduction method. Three different diameters were obtained by changing the reducing agent and the sequence of adding the materials. Particle diameter, morphology, and dispersibility were characterized using a transmission electron microscope and ultraviolet-visible (UV-vis) spectrophotometer.

Preparation of gold nanoparticles with 25 nm diameter

First, 5 mL stock solution of tetrachloroauric acid (0.2%, m/v) was added into a round-bottomed flask. After adding 91 mL ultrapure water, the mixture was violently stirred and heated to boiling, and reflux was maintained for 2 min. Next, 4 mL sodium citrate (1%, m/v) was added into the flask, and reflux was maintained for 1 h. After 1 h, the flask was naturally cooled to room temperature, sealed, and preserved at 4°C in the dark.

Preparation of gold nanoparticles with 15 nm diameter

First, 5 mL stock solution of tetrachloroauric acid (1%, m/v) was added into a round-bottomed flask. After adding 90 mL ultrapure water, the mixture was violently stirred and heated to boiling and reflux was maintained for 2 min. Next, 5 mL stock solution of tetrachloroauric acid (0.2%, m/v) was added into the flask, and reflux was maintained for 1 h. After 1 h, the flask was naturally cooled to room temperature, sealed, and preserved at 4°C in the dark.

Preparation of gold nanoparticles with 5 nm diameter

First, 5 mL stock solution of tetrachloroauric acid (0.2%, m/v) and 3 mL sodium citrate (1%, m/v) were added into a round-bottomed flask. After adding 88 mL ultrapure water, the mixture was violently stirred and heated to 60°C, and reflux was maintained for 2 min. Next, 4 mL newly prepared sodium borohydride solution (0.38%, m/v) was added and reacted for 1 h. After 1 h, the flask was naturally cooled to room temperature, sealed, and preserved at 4°C in the dark.

Preparation of aptamer-gold nanoparticle complex

Anti-TNT aptamers were dissolved in carbonate buffer (pH 9.62) to a concentration of 10−6 g/mL. Next, 1 mL gold nanoparticles were added into the centrifuge tube, and 0.1 M sodium hydroxide was used to adjust the pH to 9.5. Next, 50 µL anti-TNT aptamer solution was incubated with the gold nanoparticles at 37°C for 30 min with a rotation speed of 200 r/min. Subsequently, 100 µL BSA solution (5%, m/v) was added, mixed for 30 s, and then the mixture stood for 5 min. The tube was centrifuged at 4°C at 13,000 r/min, 14,800 ×g for 15 min. The supernatant was removed, and the precipitant was collected, dissolved in carbonate buffer (pH 9.62), and mixed well. Centrifugation was performed again under the above-mentioned conditions to remove the free aptamers and BSA. The aptamer-gold nanoparticle complexes were obtained, dissolved in 200 µL carbonate buffer (pH 9.62) as the stock solution, and preserved at 4°C. The particle diameter, morphology, and dispersibility were characterized using a transmission electron microscope and UV-vis spectrophotometer.

Construction of gold nanoparticle-enhanced surface plasmon resonance sensor

The indirect competition method was used to detect TNT in the samples. The flowchart of TNT detection using a gold nanoparticle-enhanced SPR aptamer is shown in [Figure 1]. The procedure was as follows: 1 mL stock solution of the aptamer-gold nanoparticle complex was mixed with 8 mL carbonate buffer (pH 9.62). Next, 1 mL TNT solutions of different concentration or blank buffer were added to the mixture. After mixing, the solution was incubated at 37°C for 30 min. The modified chip was installed into the detection system and detection began. Carbonate buffer (pH 9.62) was added to the sample chamber. The angle of incident light was adjusted using the mechanical rotation system, and a rough, fine angle scan was performed for the chip, with an initial scan angle of 45° and an endpoint angle of 60°; the scan step length for the rough scan was 0.2° and the step length for the fine scan was 0.05°. Based on the patterns observed from the angle scan, the resonance angle was the angle that corresponded to the smallest light intensity reflectivity. Next, software was used to determine the angle for kinetic measurement. The sampling time interval was set as 2 s, and the flow rate of the sample for the peristaltic pump was set to 400 µL/min. Next, kinetic measurement was conducted by injecting carbonate buffer (pH 9.62) into the sample chamber at 0–600 s to stabilize the baseline, and then by injecting the blank aptamer-gold nanoparticle complex or anti-TNT aptamer-gold nanoparticle complex at 600–2100 s. Binding of the free aptamer-gold nanoparticle complex to TNP-Gly on chip surface was initiated. Finally, elution was performed by injecting carbonate buffer (pH 9.62) at 2100–2700 s. Detection was finished when the response signals stabilized.


  Results and Discussion Top


Characterization of gold nanoparticles

Different nanoparticle sizes were obtained by changing the amount of trisodium citrate added. The nanoparticle size distribution was controlled by changing the reaction temperature and sequence of adding chloroauric acid and reducing agent. Sodium borohydride was used as a strong reducing agent to obtain smaller-sized nanoparticles in the presence of trisodium citrate. The experimental conditions for preparing gold nanoparticles of three different sizes are shown in [Table 1].
Table 1: Experimental conditions for preparing gold nanoparticles of three different sizes

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Gold nanoparticles prepared using the three methods were of different colors as a result of plasma resonance absorption on the nanoparticle surface. From large to small, the gold nanoparticles were pink, red, and orange-red [Figure 2]. Light absorption of the gold nanoparticles was characterized. Within the visible wavelength range, the gold nanoparticles showed a single absorption peak; as the particle size changed, the maximum absorption wavelength varied in the range of 510–550 nm. The three gold nanoparticles were characterized using a UV-vis spectrophotometer [Figure 3]. The maximum absorption wavelengths were 517.0 nm (gold nanoparticle [C]), 520.0 nm (gold nanoparticle [B]), and 525.0 nm (gold nanoparticle [A]). According to Mie, as gold nanoparticles increase in size, the maximum absorption peaks undergo a shift in the intensity of red color, which was observed in our experiment. The TEM patterns of the gold nanoparticles are shown in [Figure 4]. All three gold nanoparticles were spherical and showed a uniform size distribution. The gold nanoparticles (A), (B), and (C) were approximately 25, 15, and 5 nm, respectively.
Figure 2: Colors of the three gold nanoparticles

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Figure 3: Ultraviolet-visible spectra of the three gold nanoparticles

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Figure 4: Transmission electron microscope patterns of the three gold nanoparticles. (a) 25 nm gold nanoparticles; (b) 15 nm gold nanoparticles; (c) 5 nm gold nanoparticles

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Characterization of aptamer-gold nanoparticle complexes

The aptamer-gold nanoparticle complexes were prepared as specific recognition ligands in the SPR sensor for TNT detection. The thiol group of the anti-TNT peptide aptamer formed a self-assembled layer with the gold nanoparticles through Au-S bonding. Other studies demonstrated the high stability, orderliness, and compactness of the self-assembled layer formed through Au-S bonding.

The surface of the gold nanoparticles was modified with different groups. As a result, the resonance peaks observed in the gold nanoparticles underwent either a red shift or blue shift. This can be used to determine whether the functional groups have been attached to the gold nanoparticle. The complexes formed between 10−5 g / mL aptamers and gold nanoparticles with different sizes ([A], [B], and [C]) were detected using the UV-vis spectrophotometer. The results were compared with the spectra before modification. As shown in [Figure 5], the maximum absorption peaks in the gold nanoparticles modified by aptamers underwent an obvious red shift, indicating that the aptamers were attached to the nanoparticles. The TEM patterns of the complexes are shown in [Figure 6]. Comparison of the spectra before and after modification indicated that the aptamer-modified gold nanoparticles specifically recognized TNT and that nanoparticle dispersibility was high. This not only improves the stability of the nanoparticles but also creates the required experimental conditions. However, some nanoparticles with a smaller size (C) agglomerated. This is because smaller nanoparticles have a much higher specific surface area and therefore higher surface atom activity.
Figure 5: Ultraviolet-visible spectra of the gold nanoparticles before and after modification with aptamers. (a) 25 nm gold nanoparticles; (b) 15 nm gold nanoparticles; (c) 5 nm gold nanoparticles

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Figure 6: Transmission electron microscope patterns of aptamer-gold nanoparticle complexes. (a) Aptamer-gold nanoparticle (25 nm) complexes; (b) Aptamer-gold nanoparticle (15 nm) complexes; (c) Aptamer-gold nanoparticle (5 nm) complexes

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Working principle of gold nanoparticle-enhanced surface plasmon resonance sensor

The refractive index of the sensor chip can be changed by adsorption and dissociation of the target analytes on the chip surface. This leads to changes in the resonance angle, which can be analyzed to detect the target analytes. Because TNT has a small molecular weight (<2000 g/mol), the response signals may be too weak with the adsorption and dissociation of TNT molecule on the chip surface. Therefore, the indirect competition method is used to detect small molecular weight substances such as TNT. Before detection, the small molecular weight target analyte, or the analog of the target analyte, is immobilized to the chip surface as the receptor. Large molecular weight ligands (or those that can enhance the response signals) that specifically bind to the target analyte are added to the sample to be detected to initiate the binding until equilibrium is reached. This sample is then injected into the detection system, and free large molecular weight ligands bind to the chip surface, causing changes in the response signals. In this process, the response signals are weakened as the content of target analyte in the sample increases.

Using the indirect competition method, the SPR response signals were enhanced by the gold nanoparticles. As described in Section “Construction of gold nanoparticle-enhanced surface plasmon resonance sensor,” before formal detection, angle scanning was performed in order to modify SPR sensor and reveal the scan patterns [Figure 7]. The resonance angle of the chip is the incident angle with the smallest value of light intensity reflectivity. Under this angle, the evanescent wave caused by the total reflection has the same frequency and wave number as the surface plasma oscillation, and the intensity of the reflected light is at the lowest value. The fixed angle for kinetic measurement is then determined with software based on the resonance angle, and the light intensity reflectivity corresponding to the inflection point. The rationale is illustrated in [Figure 1]. In formal detection, the anti-TNT peptide aptamer-gold nanoparticle complexes are injected into the sensing system to initiate specific binding of the anti-TNT peptide aptamers to the TNP-Gly attached to the chip surface. When the complexes are self-assembled on the chip surface, the light intensity reflectivity increases. Strong SPR response signals are obtained after washing with the buffer [Figure 8]a. Next, under the same conditions, an appropriate amount of TNT is added to the aptamer-gold nanoparticle complexes and incubated. As the TNT specifically binds to the anti-TNT peptide aptamers, some binding sites on the complexes are blocked. This mixture is injected into the sensing system to reduce self-assembly of the complex on the chip surface. Weaker SPR response signals are obtained after elution with the buffer [Figure 8]b.
Figure 7: Angle scan patterns of the surface plasmon resonance chip

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Figure 8: Kinetic measurement of the gold nanoparticle-enhanced surface plasmon resonance sensor (a) aptamer-gold nanoparticle complexes are injected into the surface plasmon resonance sensor; (b) aptamer-gold nanoparticle complexes are first incubated with 2,4,6-trinitrotoluene before injecting into the surface plasmon resonance sensor

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Let R0 be the light intensity reflectivity corresponding to the equilibrium before injecting the sample and R be the light intensity reflectivity corresponding to the equilibrium after injecting the sample. The difference is ΔR = R − R0. Let ΔR be the difference in light intensity reflectivity between different samples at the same time point. As TNT concentration in the sample increases, the SPR response signals gradually weakened and ΔR decreased. The experimental results indicate that the SPR sensor was successfully constructed. Anti-TNT peptide aptamer-gold nanoparticle complexes are used both as specific recognition ligands and for enhancing SPR response signals.

Optimization of the size of gold nanoparticles

The size of the gold nanoparticles was optimized by first analyzing the effect of injecting the aptamers or gold nanoparticles alone on the SPR sensor signals. The experimental results are shown in [Figure 9]a. The difference in light intensity reflectivity ΔR was small before and after injection of the anti-TNT aptamers because the small molecular weight ligands attached to the chip surface only caused very weak changes to the SPR response. As shown in [Figure 9]b, a few injected gold nanoparticles remained on the chip surface, leading to small changes in the SPR response, which only minimally affected the experimental result. To maintain a fixed concentration of aptamer-gold nanoparticle complex solution, complexes formed from aptamers and gold nanoparticles of different sizes were injected. There were obvious changes in the SPR response signals because of the binding of the aptamers to the receptors attached to the chip surface and signal magnification by the gold nanoparticles; however, the signal magnification effect varied for different particle sizes. When the size of the gold nanoparticles forming the complexes with the aptamers was 25 nm [Figure 9]c or 15 nm [Figure 9]d, the SPR response signals were considerably enhanced. The light intensity reflectivity remained high after elution with buffer. Further comparison showed that the signal was weaker for the 25 nm particle, likely because of steric hindrance related to the larger particle size, which reduced binding to the chip surface. As shown in [Figure 9]e, SPR response signals fluctuated significantly when the size of the nanoparticles forming the complexes with the aptamers was approximately 5 nm. This is because some gold nanoparticles agglomerated as the aptamer-gold nanoparticle complexes were formed. Therefore, gold nanoparticles of approximately 15 nm were used to prepare the complexes.
Figure 9: Kinetic measurements of aptamers, gold nanoparticles, and aptamer-gold nanopar ticle (different sizes) complexes. (a) Anti-2,4,6-trinitrotoluene aptamer; (b) gold nanopar ticles; (c) aptamer-gold nanopar ticle (about 25 nm) complexes; (d) aptamer-gold nanoparticle (about 15 nm) complexes; (e) aptamer-gold nanoparticle (about 5 nm) complexes

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  Conclusion Top


We developed a gold nanoparticle-enhanced SPR sensor for TNT detection. The sensor uses the indirect competition method and interactions between gold nanoparticles, aptamers, and molecules attached to the chip surface. Before formal detection, the anti-TNT peptide aptamers are self-assembled on the surface of the gold nanoparticles through Au-S bonding to form aptamer-gold nanoparticle complexes. Next, TNT, the target analyte, is added to the aptamer-gold nanoparticle complexes to initiate specific binding and block some of the binding sites on the complexes. The blocked binding sites do not react with the TNP-Gly attached to the chip surface, thus weakening the SPR response and achieving indirection detection of TNT. Our method is advantageous compared with other similar sensors that have been reported previously because of its high specificity, stability, ease of preparation, and low cost. This minimized SPR sensor is suitable for fast, real-time, on-site detections such as in the field of public safety and environmental monitoring.

Acknowledgments

We are grateful to Humanities and Social Science Research Program of China University of Politics and Law (10ZFQ82009) and Academician Foundation of the Ministry of Public Security of the People's Republic of China (No. 2011-23214203, 2011-23215243), Beijing Municipal Education Commission University Science and Technology Park Construction Project (2011-08111603), and Program for Young Innovative Research Team in China University of Political Science and Law (14CXTD04, 16CXTD05), for their financial support.

Financial support and sponsorship

We are grateful to Humanities and Social Science Research Program of China University of Politics and Law (10ZFQ82009) and Academician Foundation of the Ministry of Public Security of the People's Republic of China (No. 2011-23214203, 2011-23215243), Beijing Municipal Education Commission University Science and Technology Park Construction Project (2011-08111603), and Program for Young Innovative Research Team in China University of Political Science and Law (14CXTD04, 16CXTD05), for their financial support.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9]
 
 
    Tables

  [Table 1]


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