Immunosensing of Atrazine with Antibody ... - ACS Publications

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Immunosensing of Atrazine with Antibody-Functionalized Cu-MOF Conducting Thin Films Sanjeev K. Bhardwaj,†,‡ Neha Bhardwaj,†,‡ Girish C. Mohanta,†,‡ Pawan Kumar,§ Amit L. Sharma,†,‡ Ki-Hyun Kim,*,§ and Akash Deep*,†,‡ †

Central Scientific Instruments Organisation (CSIR-CSIO), Sector 30 C Chandigarh 160030, India Academy of Scientific and Innovative Research, CSIR-CSIO, Sector 30 C, Chandigarh 160030, India § Department of Civil & Environmental Engineering, Hanyang University, 222 Wangsimni-Ro, Seoul 133-791, Republic of Korea ‡

S Supporting Information *

ABSTRACT: This work reports the assembly of thin films of a silica (SiO 2 )-modified copper−metal organic framework, Cu3(BTC)2 [Cu3(BTC)2@SiO2, BTC = benzene-1,3,5-tricarboxylic acid] on a conducting substrate of NH2−BDC [NH2−BDC = 2-aminobenzene-1,4-dicarboxylic acid] doped polyaniline (PANI). Assembled Cu3(BTC)2@SiO2/BDC-PANI thin films displayed electrical conductivity in the range of 35 μA. These thin films were conjugated with antiatrazine antibodies to create a novel immunosensing platform. Various structural and spectral characteristics of the synthesized material and its bioconjugate were investigated. The developed immunosensor was used for the conductometric sensing of atrazine. The detection of atrazine was achieved with a high sensor sensitivity (limit of detection = 0.01 nM) and specificity in the presence of diverse pesticides (e.g., endosulfan, parathion, paraoxon, malathion, and monochrotophos). KEYWORDS: MOF, conducting, thin films, sensor, polyaniline, atrazine

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been tested in conductometric gas sensing applications.18 Recently, Cu2+ and Ni2+ metal salts have been used for the development of two-dimensional 2,3,6,7,10,11-hexaiminotriphenylene (HITP)-based conductive MOFs.19 Similarly, γcyclodextrin-derived MOF and titanium-MOF (NH2-MIL-125) have been proposed for the conductometric sensing of CO2 gas and moisture content, respectively.20 The synthesis of such electrically conducting MOFs is evidence that MOF-based electronic devices could be suitable for the conductometric sensing of biomolecules as well.21,22 For their utilization in the development of biosensors, MOFs must be conjugated to biorecognition elements. Such biofunctionalization can be achieved through the use of pendant functional groups from the linker moiety of the MOF.15−17 However, this procedure does not necessarily provide control over the functionalization reaction and could lead to bulk functionalization rather than the intended surface modification. The limited aqueous solubility/dispersibility of MOFs is a significant hurdle in achieving their effective biofunctionalization.15−17 Surface modification of MOFs with a silica coating optimizes the conditions of their biofunctionaliza-

INTRODUCTION Metal organic frameworks (MOFs) are hybrid crystalline materials formed when inorganic metal ions are connected with organic linkers through strong coordination bonds. MOFs are supramolecular assemblies with unique characteristics such as high porosity, huge surface area to volume areas (500−8000 m2/g), and moderate thermal/mechanical stabilities.1−9 The use of MOFs (also known as coordination polymers (CPs)) has been explored for a wide variety of applications including gas storage and separation, catalysis, molecular sensing, guest molecule encapsulation, photovoltaics, and optics.1−7 At times, postsynthetic modification of MOFs has been used to extend their application to biosensor development, cellular imaging, and drug delivery.10−12 Recently, due to their high surface areas and oriented crystal growth, MOFs have been projected as attractive platforms for the development of highly sensitive and reproducible biosensors.13−17 Despite these advantages of MOFs, the majority of MOFbased chemo- and biosensors are based on the exploitation of the luminescence properties of MOFs for signal transduction. To tap the potential of MOFs in different fields (including biosensing), several researchers have explored the synthesis of electrically conducting MOF platforms. For example, Alterephthalate (Al-BDC), Fe-1,3,5-benzenetricarboxylate (FeBTC), and Cu-1,3,5-benzenetricarboxylate (Cu-BTC) have © XXXX American Chemical Society

Received: August 19, 2015 Accepted: November 11, 2015

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DOI: 10.1021/acsami.5b07692 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) FTIR of BDC-PANI and (b) FE-SEM of BDC-PANI film.

tion.23−25 The silica coating not only improves the water stability and dispersibility of the MOFs, but also may facilitate their effective surface functionalization. Although silica-coated MOFs have not yet been investigated thoroughly, they have been proposed as suitable candidates for imaging applications23 and for the sensing of dipicolinic acid.24 The present work for the first time reports the thin film assembly of silica-coated water-stable Cu3(BTC)2@SiO2 on a conducting substrate and its application in the sensitive conductometric sensing of atrazine (2-chloro-4-ethylamino-6isopropylamino-s-triazine), a widely used pesticide for agricultural purposes. In light of the adverse health effects of atrazine, it is important to develop sensitive and field-applicable techniques for its detection.26−32 A linker (NH2−BDC)doped polyaniline base platform has been used to assemble the thin films of Cu3(BTC)2@SiO2. These thin films have further been bioconjugated with antiatrazine antibodies. The use of this novel MOF-based immunosensor for the conductometric sensing of atrazine has thus been demonstrated in this work.

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Cu3(BTC)2@SiO2 must be treated with APTES (3-aminopropyl triethoxysilane) so that usable functional groups (−NH2) can be generated on its surface. Thus, a 100 mg sample of Cu3(BTC)2@SiO2 was incubated with 5 mL of APTES solution (1% in ethanol) for 2 h at RT. Finally, the modified Cu3(BTC)2@SiO2 was recovered and purified by successive centrifugation and washing steps. It was then washed thrice to exclude any absorbance signal of residual H3BTC linker at λ of 200−250 nm. Synthesis of NH2−BDC doped Polyaniline and the Assembly of Cu3(BTC)2@SiO2 Thin Films. The formation of the NH2−BDC doped polyaniline, achieved by mixing 10 mL of 0.1 M aniline (in 1 M HCl) and 2.5 mL of a water-dispersed sodium salt of 0.1 M NH2− BDC (2-amino terephthalic acid), was performed according to a previously published protocol.15,17 The reaction contents were placed in an ice bath, and 0.1 M ammonium persulfate was added dropwise. The oxidation of aniline monomers resulted in the formation of NH2− BDC doped polyaniline (BDC-PANI). The reaction mixture was left overnight to allow the reaction to complete. Then, the BDC-PANI was separated and purified through successive centrifugation and washing steps (n = 3). These cleaning steps were also necessary to eliminate any absorbance signal of residual linker or PANI at λ of 200−325 nm. The obtained BDC-PANI material was used to design a conducting surface for the further assembly of Cu3(BTC)2@SiO2 films. For this, a suitable amount of BDC-PANI was mixed with water to make a slurry, which then was spin-casted onto a four-electrode sensor surface. The BDC-PANI electrode (with the available −COOH groups from BDC) was left to incubate (30 min) with a reaction solution of 0.05 M EDC [(1-ethyl-3-(3-(dimethylamino)propyl) carbodiimide] + 0.01 M NHS (N-hydroxysuccinimide). This was followed by the spin-casting of an aqueous suspension (50 mg/mL) of Cu3(BTC)2@SiO2 on the BDCPANI electrode. This assembled thin film of Cu3(BTC)2@SiO2/BDCPANI was annealed at 100 °C to ensure its robust adhesion. A schematic of the whole process is shown in Figure S1 (Supporting Information). Immobilization of Antiatrazine Antibodies on Cu3(BTC)2@ SiO2/BDC-PANI Electrode. To begin, 100 μL of the antiatrazine antibody solution (1 μg/mL) was left to incubate for 30 min with 50 μL of EDC (0.05 M)-NHS (0.01 M) prepared in MES (2-Nmorpholino ethanesulfonic acid) buffer (pH 5.0). Then, this antibody solution was left in homogeneous contact with the Cu3(BTC)2@SiO2/ BDC-PANI electrode for 30 min to allow the formation of the antibody-immobilized immunosensor. Finally, the immunosensor was washed with water and treated with a 0.01% Tween 20 solution (in phosphate-buffered saline, PBS, pH 7.4) in order to block nonspecific binding sites. Analysis of Atrazine in Spiked Water Samples and Validation. As a test of its practical performance, the developed immunosensor was employed to quantify atrazine in water samples spiked with atrazine at five different concentration levels (from 10 pM to 10 nM; Table S1). The results were validated by a parallel HPLC analysis across the selected concentration range. Before measurements, all samples were filtered with a 0.22 μm PTFE membrane. The HPLC

EXPERIMENTAL DETAILS

Materials. A monoclonal antiatrazine antibody (produced in mouse; concentration, 2.51 mg/mL) was purchased from Abcam, India. Atrazine and all other reagents/solvents employed in this study were of analytical-grade purity from Sigma-Aldrich/Pierce/Merck. Fourier transform infrared spectroscopy (FTIR, Nicolet, iS10) was used to confirm the presence of the silica coating on the MOFs. Morphological studies were carried out with a field emission scanning electron microscope (FE-SEM, Hitachi S4300 SE/N). The X-ray diffraction patterns of the samples were recorded on an XPert Powder3 system (PANalytical). Nitrogen isotherm analysis for the determination of the surface area and pore volume was done with a BELSORP-Max system. The current−voltage (I−V) parameters were studied with a semiconductor characterization system (Keithley Model 4200). Synthesis of the MOF. The silica-coated Cu-MOF [Cu3(BTC)2@ SiO2] was synthesized using benzene-1,3,5-tricarboxylic acid (H3BTC) (98%) as an organic linker. Briefly, 0.5 mmol H3BTC and 1.6 mmol sodium hydroxide (NaOH) solutions were dissolved in a mixture of 2 mL N,N′-dimethylformamide (DMF), 20 mL of deionized (DI) water and 15 mL ethanol. Then, 5 mL of 0.5 mmol Cu(OAc)2·H2O was added to the above reaction mixture. The resulting mixture (in a 500 mL reagent bottle) was ultrasonicated at room temperature (RT, 25 °C) for 30 min, and then 4.5 mmol tetraethylorthosilicate (TEOS) was added dropwise. The reaction mixture was subjected to further ultrasonication for 2 h, and the obtained Cu3(BTC)2@SiO2 particles were purified by centrifugation (12000 rpm), washed with 30% ethanol, and dried (80 °C) under a vacuum. Note that the synthesized B


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Figure 2. (a) FTIR of Cu3(BTC)2, Cu3(BTC)2@SiO2, and antibody/Cu3(BTC)2@SiO2 and (b) FE-SEM of Cu3(BTC)2@SiO2 thin films on BDCPANI.

Figure 3. (a) XRD patterns of Cu3(BTC)2 and Cu3(BTC)2@SiO2 and (b) N2 absorption−desorption isotherms of Cu3(BTC)2@SiO2. chromatographic conditions were as follows: column, C18; column temperature, 25 °C; mobile phase, 70% methanol; flow rate, 1.0 mL/ min; detection wavelength, 224 nm.

FTIR of the BDC-PANI. The bands at around 1580 and 1490 cm−1 correspond to stretching vibrations of benzenoid and quinoid rings. A cluster of bands appeared between 1450 and 1150 cm−1 due to the aromatic C−N stretching of polyaniline. The doping of BDC in PANI was confirmed by the characteristic bands of the nonionized carboxyl group of the ligand (ν−OH, 3135 cm−1 and νCO, 1700 cm−1) and the stretching vibrations of its carboxylate ions (1620−1550 and 1400−1350 cm−1). The FE-SEM image of the BDC-PANI film of the four-electrode device is shown in Figure 1b. These data reveal that BDC-PANI films formed a homogeneous base with nanofibrous structures having diameters of around 100 nm. The FTIR data for Cu3(BTC)2, Cu3(BTC)2@SiO2, and antibody/Cu3(BTC)2@SiO2 samples are presented in Figure 2a. In the FTIR spectrum of Cu3(BTC)2, prominent peaks at 1618 and 1560 cm−1 were attributed to the asymmetric stretching of the carboxylate groups of the BTC linkers. The bands appearing at 1419 and 1369 cm−1 also may have arisen due to symmetric stretching of carboxylate groups. A peak at 757 cm−1 may reflect the bending vibrations of C−H of the linker molecule. The bands at around 930, 3440, and 3000− 2500 cm−1 may have appeared due to the bending and stretching frequencies of the O−H group of ethanol (or absorbed water molecules). The FTIR spectrum of Cu3(BTC)2 was in good agreement with the spectra in the reported literature.33−35 The FTIR data for Cu3(BTC)2@SiO2 were similar to those of Cu3(BTC)2, with an additional peak around 1100 cm−1 attributed to Si−O−Si stretching.33−35 In the FTIR

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RESULTS AND DISCUSSION Characterization of the Synthesized Immunosensor. A typical synthesis approach was used to form Cu-MOF nanocrystals, and the subsequent SiO2 coating led to the formation of water-dispersible Cu3(BTC)2@SiO2. It was noticed that the obtained dispersion was stable even after being left overnight. After APTES treatment of the Cu3(BTC)2@SiO2, the silane ends were covalently bound to the Si surface, while the −NH2 groups on the terminal end facilitated the formation of a thin film of the MOF on the BDCPANI surface, as well as the binding of antibodies. The bioconjugation of the APTES-modified Cu3(BTC)2@SiO2 thin films with the antiatrazine antibodies was performed in MES medium, which is a nonamine and noncarboxylate buffer and does not interfere with the desired formation of an amide bond between the −NH2 groups of the MOF and −COOH groups of the biomolecule. The copolymerization of NH2−BDC and PANI was achieved by oxidation with ammonium persulfate. NH2−BDC is an attractive option for the synthesis of −COOH functionalized PANI, as it contains two −COOH groups linked to an aromatic ring. This kind of self-doping eliminates the need for an external dopant. The schematic of copolymerization of NH2− BDC and PANI is depicted in Figure S2. Figure 1a shows the C


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Figure 4. (a) Current−voltage (I−V) properties of Cu3(BTC)2 and Cu3(BTC)2@SiO2 and (b) I−V properties of Cu3(BTC)2@SiO2/BDC-PANI.

be in the range of nS, whereas those from Cu3(BTC)2@SiO2/ BDC-PANI films were around 35 μS. This useful improvement in the conductivity of the latter was exploited for the immunosensing of atrazine. Figure 5 shows the change in conductance after the immobilization of antiatrazine antibodies on the Cu3(BTC)2@SiO2/BDC-PANI electrode, and the subsequent response of this immunosensor to varying atrazine concentrations (from 0.0001 nM to 10 μM). As expected, the conductance of the antibody/Cu3(BTC)2@SiO2/BDC-PANI sensor decreased slightly after the immobilization of the antibody. This decrease in the sensor’s conductance should be attributed at least partially to the poor diffusion of electrolyte molecules toward the sensor surface in the presence of the biomolecule coating over the Cu3(BTC)2@SiO2/BDCPANI surface. Different concentrations of the pesticide atrazine (0.0001 nM − 10 μM) were analyzed with the antibody/Cu3(BTC)2@ SiO2/BDC-PANI sensor. The developed current and its changes (with respect to a fixed bias potential of 1 V) were correlated with the tested pesticide concentrations. All results are presented as the average of triplicate measurements (with error bars in Figure 5). The conductance values of the sensor decreased proportionately to the pesticide concentrations. Based on an extensive investigation of the response of the immunosensor to different atrazine concentrations, the dynamic detection range of the proposed immunosensor was estimated as 0.01 nM−1 μM. The plotted S-shaped logarithmic curve (Figure 5b) has been analyzed with Four-Parameter Logistic (4-PL) regression method with the following derivative equation.

spectrum of antibody/Cu3(BTC)2@SiO2, the characteristic peaks of Cu3(BTC)2 were largely masked, and instead a broad peak was observed at around 1640 cm−1, which is characteristic of protein (antibody) amide bonds.33−35 The appearance of an important Cu−O stretch peak at around 700 cm−1 demonstrates that the MOF structures with Cu and BTC molecules remained stable during the silica coating and antibody immobilization steps. Figure 2b displays the FE-SEM image of Cu3(BTC)2@SiO2 thin films on the BDC-PANI surface. The crystallite structure of Cu3(BTC)2@SiO2 is evident. The size of the MOF crystals was in the range of 100−200 nm. It is also clear that the assembly of Cu3(BTC)2@SiO2 was fairly homogeneous. Figure 3a shows the XRD patterns for Cu3(BTC)2 and Cu3(BTC)2@ SiO2. The pattern for the synthesized MOF was consistent with those in the literature.33−35 The coating of Cu3(BTC)2 with SiO2 reduced its diffraction intensity, while the crystal structure remained stable. Studies of the BET surface area indicated that the Langmuir surface area of the Cu3(BTC)2@SiO2 was 238 m2/g. The absorption−desorption isotherms (Figure 3b) were typical of mesoporous materials. This surface area value of Cu3(BTC)2@SiO2 was lower than those reported previously: (1500−2100 m2/g) of Cu3(BTC)2.33−35 This reduction in the surface area of the MOF was linked with the wrapping of the MOF molecules with SiO2. However, in both cases, the observed pore volume was 0.33 cm3/g. This observation suggests that the modification of the MOF with the SiO2 layer did not alter its intrinsic porosity. Performance of the Synthesized Immunosensor. The electrical conductivity data (I−V) curves for Cu3(BTC)2, Cu3(BTC)2@SiO2, and Cu3(BTC)2@SiO2/BDC-PANI, when exposed to a water sample, are depicted in Figure 4. All these data were collected for the thin films assembled on the fourprobe electrode system as shown in Figure S1. The flows of current from Cu3(BTC)2 and Cu3(BTC)2@SiO2 were found to

y=d+

a−d ⎡ a b⎤ ⎢⎣1 + c ⎥⎦

()

D

(1) DOI: 10.1021/acsami.5b07692 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article


Table 1. Comparison of Different Sensors Reported for the Analysis of Atrazine detection platform gold nanoparticles SSA-PAni ZnPP gold nanoparticles CNT GNP/MWCNT composite nano CuO electropolymerized poly(pyrrolenitrilotriacetic acid) film microcantilever atrazine-HRP/Ab/ProtAGEB atrazine/atrazineHRPscAb/PANI/ PVSA/SPE poly(JUG-HATZ) interdigitated microelectrodes (IDμE) dipstick gold/MHDA+biotinyl-PE antibody/Cu3(BTC)2@ SiO2/BTC-PANI

technique

detection limit (nM)

ref

electrochemical impedimetric sensing molecularly imprinted polymers electrochemical immunosensor electrochemical impedance spectroscopy EIS

0.46 0.05

27 30

20

36

0.07

37

0.46

38

0.02

39

square wave voltammetry cyclic voltammetry

9.27

40

0.05

41

amperometric

0.005 27820

43 44

amperometric

0.46

45

square wave voltammetry impedimetric

0.001

42

0.88

46

immunoassay impedimetric conductometry

0.46 92.7 0.01 (2.15 ng/L)

47 48 this work

the antibody/Cu3(BTC)2@SiO2/BDC-PANI immunosensor was monitored as it was incubated with a suitable aliquot of the above possible interfering pesticides at a fixed concentration of 1 μM. No significant change in the base conductance of the immunosensor was detected during the test with nonspecific pesticides (Figure S3). Thus, the practical utility of this technique was validated in terms of specificity. The response time was checked in a test with a 1 μM atrazine sample. The measurements of current at a bias potential of 1 V started as soon as the analyte sample was introduced onto the surface of the sensor. The results shown in Figure S4 highlight the response time of the sensor to allow a stable reading for around 3 min. Even beyond that, no particular change was observed in the readings. The reproducibility of the immunosensor also was investigated through intra- and interassay precision measurements. For intra-assay precision, the response of the sensor to 1 μM atrazine was checked periodically. The readings were almost stable (±5%) over a period of 3 weeks (Figure S5). The interassay precision also was assessed by measuring one concentration of the atrazine sample using four separately prepared sensor electrodes. In this case, the variation in the response of the sensors was limited to ±4% (Figure S6). The results of spiked water analysis using the antibody/ Cu3(BTC)2@SiO2/BDC-PANI immunosensor are presented in Table S1 (Supporting Information). The recoveries of atrazine from both the immunosensor and HPLC closely match with the known spiked concentration. The HPLC analysis also validated the reliability of the atrazine detection by the immunosensor proposed in this work.

Figure 5. (a) Response of the antibody/Cu3(BTC)2/BDC-PANI immunosensor to atrazine (0.0001 nM-10 μM); (inset) change in the conductance of Cu3(BTC)2@SiO2/BDC-PANI thin films after immobilization of the antiatrazine antibodies; and (b) logarithmic response curve of the immunosensor with respect to varying concentrations of atrazine, 4-PL fitting equation, and the estimated parameter values.

Here a, b, c, and d represent the response at zero concentration, slope, midrange concentration, and response at infinite concentration, respectively. The equation for the backestimations from the calibration curve is mentioned in Figure 5b. The data were fitted with a free trial version of AssayFit excel add-in. The optimized values of parameters a, b, c, and d are indicated in Figure 5b. The response of the sensor at higher test concentrations was saturated due to the lack of probe molecules over the surface of the sensor. The above-mentioned concentration-dependent reduction in the conductance of the sensor can be accounted for by the insulating nature of the atrazine molecules, which, by binding with the immobilized antibodies, acted as a definite kinetic barrier for electron transport. The primary objective in designing the proposed immunosensor was to detect the lowest possible atrazine concentration, and the system displayed excellent performance in this regard. The limit of detection (LOD) was estimated as 0.01 nM, which is an excellent achievement among all the preexisting techniques (Table 1). The proposed immunosensor was tested against several other (nonspecific) pesticides, namely, endosulfan, parathion, paraoxon, malathion, and monochrotophos. The conductance of E


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(5) Barea, E.; Montoro, C.; Navarro, J.-A. Toxic Gas Removal− Metal-Organic Frameworks for the Capture and Degradation of Toxic Gases and Vapours. Chem. Soc. Rev. 2014, 43, 5419−5430. (6) DeCoste, J.-B.; Peterson, G.-W. Metal-Organic Frameworks for Air Purification of Toxic Chemicals. Chem. Rev. 2014, 114, 5695− 5727. (7) Kumar, P.; Deep, A.; Kim, K.-H.; Brown, R. J. Coordination polymers: Opportunities and Challenges for Monitoring Volatile Organic Compounds. Prog. Polym. Sci. 2015, 45, 102−118. (8) Kumar, P.; Kim, K.-H.; Deep, A. Recent Advancements in Sensing Techniques based on Functional Materials for Organophosphate Pesticides. Biosens. Bioelectron. 2015, 70, 469−481. (9) Kumar, P.; Deep, A.; Kim, K.-H. Metal organic frameworks for sensing applications. TrAC, Trends Anal. Chem. 2015, 73, 39−53. (10) Tanabe, K.-K.; Cohen, S.-M. Postsynthetic Modification of Metal−Organic FrameworksA Progress Report. Chem. Soc. Rev. 2011, 40, 498−519. (11) Jung, S.; Kim, Y.; Kim, S.-J.; Kwon, T.-H.; Huh, S.; Park, S. BioFunctionalization of Metal−Organic Frameworks by Covalent Protein Conjugation. Chem. Commun. 2011, 47, 2904−2906. (12) McKinlay, A.-C.; Morris, R.-E.; Horcajada, P.; Férey, G.; Gref, R.; Couvreur, P.; Serre, C. BioMOFs: Metal−Organic Frameworks for Biological and Medical Applications. Angew. Chem., Int. Ed. 2010, 49, 6260−6266. (13) Shekhah, O.; Liu, J.; Fischer, R.; Wöll, C. MOF thin films: existing and future applications. Chem. Soc. Rev. 2011, 40, 1081−1106. (14) Doherty, C. M.; Grenci, G.; Riccò, R.; Mardel, J. I.; Reboul, J.; Furukawa, S.; Kitagawa, S.; Hill, A. J.; Falcaro, P. Combining UV Lithography and an Imprinting Technique for Patterning MetalOrganic Frameworks. Adv. Mater. 2013, 25, 4701−4705. (15) Deep, A.; Bhardwaj, S. K.; Paul, A.; Kim, K.-H.; Kumar, P. Surface Assembly of Nano-Metal Organic Framework on Amine Functionalized Indium Tin Oxide Substrate for Impedimetric Sensing of Parathion. Biosens. Bioelectron. 2015, 65, 226−231. (16) Kumar, P.; Deep, A.; Paul, A.; Bharadwaj, L. Bioconjugation of MOF-5 for Molecular Sensing. J. Porous Mater. 2014, 21, 99−104. (17) Kumar, P.; Kumar, P.; Bharadwaj, L.-M.; Paul, A.; Deep, A. Luminescent Nanocrystal Metal Organic Framework based Biosensor for Molecular Recognition. Inorg. Chem. Commun. 2014, 43, 114−117. (18) Limit of thermal stability according to thermogravimetric analyses: (a) Basolite A100 (Al-BDC), > 400 °C; (b) Basolite C300 (Cu-BTC, Cu3BTC2), 320 °C; (c) Basolite F300 (Fe-BTC), 310 °C. (19) Sheberla, D.; Sun, L.; Blood-Forsythe, M.-A.; Er, S.; Wade, C.R.; Brozek, C.-K.; Aspuru-Guzik, A.; Dincă, M. High Electrical Conductivity in Ni3 (2, 3, 6, 7, 10, 11-hexaiminotriphenylene) 2, a Semiconducting Metal−Organic Graphene Analogue. J. Am. Chem. Soc. 2014, 136, 8859−8862. (20) Han, S.; Wei, Y.; Valente, C.; Forgan, R.-S.; Gassensmith, J.-J.; Smaldone, R.-A.; Nakanishi, H.; Coskun, A.; Stoddart, J.-F.; Grzybowski, B.-A. Imprinting Chemical and Responsive Micropatterns into Metal−Organic Frameworks. Angew. Chem., Int. Ed. 2011, 50, 276−279. (21) Givaja, G.; Amo-Ochoa, P.; Gómez-García, C. J.; Zamora, F. Electrical conductive coordination polymers. Chem. Soc. Rev. 2012, 41, 115−147. (22) Hendon, C.-H.; Tiana, D.; Walsh, A. Conductive Metal− Organic Frameworks and Networks: Fact or Fantasy? Phys. Chem. Chem. Phys. Chem. Chem. Phys. 2012, 14, 13120−13132. (23) Taylor-Pashow, K.-M.; Rocca, J.-D.; Xie, Z.; Tran, S.; Lin, W. Postsynthetic modifications of Iron-Carboxylate nanoscale Metal− Organic frameworks for Imaging and Drug Delivery. J. Am. Chem. Soc. 2009, 131, 14261−14263. (24) McGuire, C.-V.; Forgan, R.-S. The surface chemistry of metal− organic frameworks. Chem. Commun. 2015, 51, 5199−5217. (25) Rieter, W.-J.; Taylor, K.-M.; Lin, W. Surface Modification and Functionalization of Nanoscale Metal-Organic Frameworks for Controlled Release and Luminescence Sensing. J. Am. Chem. Soc. 2007, 129, 9852−9853.

CONCLUSIONS The work reported herein has demonstrated that the binding of MOF films on conducting surfaces can facilitate the development of effective immunosensors. In comparison to other advanced nanostructured platforms (e.g., graphene and carbon nanotubes (CNTs)), MOF-based devices may demonstrate comparable or even improved performance. The high surface areas of MOFs are a particular advantage. Efficient bioconjugation of receptor molecules with MOFs has been considered a bottleneck in realizing their application in the biosensing field. In this study, the approach used for the silica coating of a Cu3(BTC)2 MOF yielded attractive results in facilitating the optimal synthesis of an antibody-loaded MOF bioprobe. Another major contribution of this study was the demonstration that a conducting PANI layer can be used as a base platform for the development of MOF thin films. These MOF thin films have been used for the immunosensing of atrazine pesticide with a very low limit of detection at 0.01 nM.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07692. Additional information including the schematic of the immunosensor, specificity studies, response time, and sensor stability. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +82 2220 2325. Fax: +82 2 2220 1945. *E-mail: [email protected]. Tel: 0172-2657811, ext 452. Fax: 0172-2657287. Author Contributions

The manuscript was written with contributions from all authors. All authors have approved the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded through an India grant (OMEGA/ PSC0202/2.2.5) from CSIR, India. We are grateful to the Director of CSIR-CSIO, Chandigarh, India for supporting this research. P.K. and K.H.K. also acknowledge a financial grant from the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (MEST; No. 2009-0093848).

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REFERENCES

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