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Covalent functionalization of zinc oxide nanowires for high sensitivity p-nitrophenol detection in biological systems
Materials Science and Engineering B 177 (2012) 1583–1588
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Covalent functionalization of zinc oxide nanowires for high sensitivity p-nitrophenol detection in biological systems Anurag Gupta a,∗ , Bruce C. Kim a , Eugene Edwards b , Christina Brantley b , Paul Ruffin b a b
The University of Alabama, 101 Houser Hall, Tuscaloosa, AL 35487, USA U.S. Army, RDECOM/AMRDEC, 5400 Fowler Road, Redstone Arsenal, AL 35898, USA
a r t i c l e
i n f o
Article history: Received 15 March 2012 Received in revised form 14 June 2012 Accepted 13 August 2012 Available online 5 September 2012 Keywords: Zinc oxide nanowire Surface functionalization 1-Pyrenebutyric acid Fluorescent sensing
a b s t r a c t High-quality zinc oxide (ZnO) nanowires were synthesized using the atmospheric chemical vapor deposition technique and were appropriately characterized. Subsequently, the nanowire surface was covalently grafted with 1-pyrenebutyric acid (PBA) fluorophore, and surface-sensitive X-ray photoelectron spectroscopy and Fourier transform infrared-attenuated total reflectance spectroscopy were utilized to confirm the functionalization of 1-pyrenebutyric acid on the nanowire surface. Additionally, photoluminescence (PL) measurements were used to evaluate the optical behavior of pristine nanowires. Through fluorescence quenching of 1-pyrenebutyric acid by p-nitrophenol, a detection limit of 28 ppb was estimated. Based on these findings, ZnO nanowires functionalized with 1-pyrenebutyric acid are envisaged as extremely sensitive platforms for the ultra-trace detection of p-nitrophenol in biological systems. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nitrophenols are a class of organic compounds generated as degradation products of many organophosphorus compounds (OPs). OPs are key components of modern agricultural products such as herbicides and insecticides. Although these compounds do not accumulate in biological systems, they exhibit high toxicity and are purported to have carcinogenic effects on human subjects [1]. Empirical evidence suggests that these OPs have neurotoxic effects in that they inhibit enzymatic activity related to nerve transmission [2]. Furthermore, p-nitrophenol has been found to be an environmental contaminant in surface water and can be corrosive and acutely toxic when ingested orally, resulting in headaches, nausea and cyanosis [3,4]. Therefore, detection of p-nitrophenol is a major biological and environmental concern, and it is crucial to find simple and straightforward methodologies for ultratrace detection. Previous investigations have developed numerous detection methodologies for trace sensing of p-nitrophenols. Conventional methods include UV–vis spectrophotometric methods [5], high performance liquid chromatography (HPLC) [6], molecular imprinted polymer solid phase extraction [7] and optical membrane based sensors [8–10]. Although all these methods are capable of sensitive detection, they are complicated and thus have limited potential for large-scale sensor development due to cost and
∗ Corresponding author. Tel.: +1 205 764 3925. E-mail address: [email protected] (A. Gupta). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.08.002
other practical limitations. Hence, an approach that ensures costeffective ultratrace detection remains to be found. Fluorescence-based methods offer a prospective solution in terms of detection capabilities and have been shown to detect ppb levels of analyte. Fluorophores have enabled researchers to study biological phenomenon in living tissues and cells. When used in isolation or appropriately tagged with receptors, they can detect specific binding events with molecular-level precision, providing fundamental information. However, the use of fluorescent probes can be extremely limited for several reasons. First, the toxicity of fluorophore to biological systems severely limits the choices available for a particular application. Consequently, identifying a fluorophore that can sense a particular analyte within a ppb level detection limit and is biologically compatible is an extremely challenging task. For example, Paliwal et al. have reported a coumarin-based fluorescent p-nitrophenol sensor with a detection limit of 128 M, which is insufficient for nanomolar- or picomolar-level detection [11]. Secondly, it is likely that the identified fluorophore may not offer flexibility in detecting a spectrum of compounds and may be limited to a particular analyte. This could be a bottleneck to large-scale biosensor fabrication, where robust biosensing platforms capable of sensing multiple compounds are required. Finally, the fluorophore required must bind reversibly to the analyte. This ensures that the fabricated biosensor can be used for multiple cycles and be refreshed if needed. The aforementioned challenges can be bypassed through the design of novel sensing platforms and the utilization of identified fluorophores for enhanced signal-to-noise by functionalizing them onto appropriate, biologically compatible substrates. This can significantly
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increase the sensitivity of a fluorophore while offering flexibility in terms of sensor design. The development of high surface area nanostructures is currently an area of extensive research, particularly with respect to applications in the fields of catalysis, gas storage and sensor technology [12,13]. Organic functionalized nanowire arrays are therefore an excellent choice for biosensing platforms as their large surface area can potentially aid in an extremely sensitive detection of analyte. This surface-related phenomenon coupled with quantum confinement effects, results in excellent optical and electrical properties that can be tuned to address specific needs. Furthermore, the porous structure of these nanowires facilitates the rapid diffusion of analyte molecules to binding sites, leading to a faster response rate in the sensor. Zinc oxide (ZnO) nanowires, in addition to demonstrating aforementioned advantages, can be grown cost-effectively over an insulating substrate. This facilitates device fabrication as orientation of ZnO nanowires can be easily controlled to yield conductive mesh-like structure over the substrate itself, thereby reducing processing costs. Furthermore, ease of tuning the optical properties through precise control of nanowire diameter might provide additional advantages in terms of a hybrid opto-electronic platform for actual biosensor fabrication. Accordingly, this study involves the use of ZnO nanowires, which have previously shown very good potential in terms of aiding optical detection of biomolecules [14] along with other applications related to nanocatalysis, photovoltaics, OLED-based displays and chemical gas sensors [15–18]. Furthermore, functionalized ZnO nanowires have been demonstrated as excellent sensing platforms for, both chemical and biological sensing. SelegArd et al. reported two-step functionalization process for ZnO nanoparticle functionalization [19]. High affinity for biotin was reported using the functionalized nanostructures with potential applications in biosensing and biological imaging. Similarly, Ansari et al. [20] along with Choi et al. [21] reported biosensing platforms using functionalized ZnO nanowires. In this work we demonstrate the synthesis of a ZnO nanowire heterostructure, that has been covalently functionalized with pyrenebutyric acid (PBA) for prospective trace p-nitrophenol detection in biological systems. The ZnO nanowires used for functionalization are synthesized through a simple and straightforward atmospheric chemical vapor deposition (ACVD) process. Appropriate characterization studies along with evidence to support the hypothesis of ultrasensitive detection using PBA are also presented. We then outline the possible mechanism of sensing through these functionalized nano-heterostructures.
2. Materials and methods 2.1. Synthesis of ZnO nanowires ZnO nanowires were synthesized on silicon (1 0 0) substrate (MTI-Corp.) using vapor–liquid–solid technique. Briefly, ZnO (99.9%, J T Baker) powder and graphite powder (99.99%, 300 mesh, Alfa Aesar) were mixed in equal proportion and loaded onto a silica boat to serve as the precursor vapor source for the chemical vapor deposition process. The boat loaded with this powder mixture was introduced in a customized CVD furnace operating at atmospheric pressure. Silicon (1 0 0) substrates, ultrasonically cleaned for 30 min to remove contaminants from the surface, were subsequently coated with a thin layer of gold (ca. 4 nm) to serve as a catalyst during the growth process and were kept downstream in the furnace. Carrier gas, a mixture of argon and oxygen, was introduced from one end of the furnace while the temperature of the furnace was raised to 950 ◦ C. At this high temperature, the gold layer transformed to gold nanodots, which served as nucleation
sites for ZnO nanowire growth. The zinc vapors generated by the reduction of ZnO were carried to the substrate by the carrier gas, where the supersaturation of gold nanodots on the silicon (1 0 0) led to the growth of ZnO nanowire through the vapor–liquid–solid (VLS) mechanism [22]. A growth time of 30 min was allowed. Subsequently, the substrates were removed after furnace cooling and were observed to be covered with a gray coating. 2.2. Functionalization of ZnO nanowires The substrates coated with pristine nanowires were dipped in a 10−3 M solution of the fluorophore, 1-pyrenebutyric acid (VWR corp.), prepared in acetone and subjected to mild agitation with a common laboratory shaker for 1 h. Thereafter, the samples were immersed in a fresh solution of 1-pyrenebutyric acid in acetone, of the same concentration and were re-agitated for 1 h. This process was repeated three times to ensure uniform and complete coating of the nanowires with 1-pyrenebutyric acid. Subsequently, the samples were copiously rinsed for 5 min in pure solvent to remove physisorbed or free fluorophore, were blow-dried gently with nitrogen and were stored in a dry box for 1 week to ensure complete solvent evaporation. It is to be noted that functionalization of ZnO nanowires with carboxylic acid moiety has been demonstrated in previous reports [23,24]. Long chain organic acids form stable covalent bonds on nanowire surface instead of chemically damaging the surface. 2.3. Characterization and fluorescence studies The morphology and composition of the synthesized ZnO nanowires were characterized using field emission-scanning electron microscopy (FE-SEM, JEOL 7000) and energy dispersive X-ray analysis (EDAX, Oxford Instruments). For analyzing crystal structure transmission electron microscopy (TEM, FEI-Technai) equipped with a LaB6 source operated at 200 kV and X-ray diffraction (XRD, Bruker AXS) with Cu-K␣ radiation was used. The surface modification of the pristine nanowires was probed using the surface-sensitive technique of X-ray photoelectron spectroscopy (XPS, Kratos Axis 165 XPS/Auger). XPS is highly efficient in providing surface information and quantification, and it has depth resolution of sub-nm in terms of information it can provide from the surface. The survey scan was performed at a pass energy of 160 eV; for high-resolution scans, a pass energy of 40 eV was utilized. To ensure covalent bonding, complementary Fourier transform infrared-attenuated total reflectance (FTIR-ATR) measurements were obtained from PerkinElmer Spectra 100 equipped with diamond crystal tip. UV–vis measurements were taken on a Cary 50 spectrometer while the fluorescence data were obtained on a FluoroMax-3 (Jobin Yvon) spectrophotometer equipped with a 150 W Xe lamp. A standard 1 cm quartz cuvette was employed to ensure transparency in the UV region of the spectrum. 3. Results and discussion The morphology of the pristine ZnO nanowires was confirmed through FE-SEM investigations as shown in Fig. 1(a) and (b). The synthesized nanowires were 60–80 nm in diameter and 3–5 m in length. We also observed that the synthesized nanowires were randomly oriented and lacked any vertical alignment with the substrate. This is believed to be the result of high lattice mismatch between the silicon substrate and ZnO. Furthermore, Fig. 1(b) clearly shows that in addition to high aspect ratio growth, small percentages of the ZnO nanowires exhibit lateral growth. This can be attributed to a high supersaturation of zinc vapor in the tube furnace or to the depletion of catalyst from growth fronts [25]. We believe that in our case the latter effect played a dominant role
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Fig. 2. X-ray diffraction spectrum of pristine ZnO nanowires. Fig. 1. (a), (b) FE-SEM micrographs of as-synthesized ZnO nanowires, (c) EDAX spectrum of assynthesized.
as gold catalyst nanoparticles were absent from the tips of the nanowires. Through EDAX analysis, as shown in Fig. 1(c), it was determined that no contaminant phases were present in the synthesized nanowires. Furthermore, the atomic percentages of zinc and oxygen were found to be in agreement with the stoichiometric ratio of the molecule, which indicated that ZnO was the only constituent of the synthesized nanowires. The orientational preference of the synthesized nanowires was determined through XRD investigations as shown in Fig. 2. It can be seen that nanowires tend to be oriented along (1 0 1) plane, with a significant proportion oriented along (0 0 2) plane. This significant proportion is indicative of wurtzite phase of ZnO nanowires. To confirm hexagonal wurtzite phase and single crystalline nature of ZnO nanowires, TEM analysis was accordingly performed. Fig. 3(a) shows a typical low magnification image of a ZnO nanowire, which is completely consistent with FE-SEM investigations. The inset of Fig. 3(a) shows the selected area electron diffraction (SAED) pattern, which confirms the high crystallinity of
the synthesized nanowires along with a wurtzite-type structure. Furthermore, Fig. 3(b) presents the HR-TEM (high resolution-TEM) image of the pristine nanowire, which conforms with empirical observations of ZnO nanowires’ tendency toward c-axis growth direction via a vapor–liquid–solid process and typical interplanar spacing of 0.52 nm. To determine the optical characteristics and estimate the defect concentration of the pristine nanowires, photoluminescence (PL) was measured. The resulting PL curve is shown in Fig. 4. The spectrum was found to be clearly dominated by the emission peak at 380 nm (termed as NBE or near band-edge emission peak), originating from the radiative recombination of donor-bound excitons. Additionally, the defect-related peak centered around 500 nm (termed as DL or deep-level emission peak) is extremely weak. Although the exact origin of this deep-level emission is still unclear, it is, however, generally accepted that it potentially stems from oxygen-related vacancy defects [26]. Since in our case this defectrelated peak is almost non-existent, it can be safely deduced that the synthesized nanowires are practically defect free. This observation is particularly important as high-quality, defect-free
Fig. 3. (a) Low resolution-TEM image of a single ZnO nanowire (inset: SAED pattern from the nanowire), (b) high resolution-TEM image showing lattice spacing in growth direction.
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Fig. 4. Photoluminescence spectrum of pristine ZnO nanowires showing band-edge emission.
nanowires can potentially aid in the enhancement of optical response by suppressing the charge transfer to defect sites, thereby affecting the quantum yield of the fluorophore while simultaneously increasing the sensitivity. As mentioned previously, we used 1-pyrenebutyric acid as the fluorophore because it can be grafted onto the ZnO nanowire surface and could potentially provide ultrasensitive detection capabilities for p-nitrophenol present in biological systems. This choice was also based on Lianos and Cremel’s [27] finding that pyrenebased compounds can be used for probing biological structures. Additionally, these compounds’ nitroaromatic sensing capabilities on ruthenium nanoparticles have been independently evaluated by Chen et al. [28]. To determine that the pristine ZnO nanowires had been covalently functionalized by 1-pyrenebutyric acid, we utilized the XPS technique. XPS is a highly surface-sensitive technique that yields spectral information from the surface with sub-nm resolution, thereby making it an appropriate method to determine the surface functionalization of the pristine nanowires. The survey scan from the pristine nanowire sample shown in Fig. 5(a) indicates the presence of the following elements: Zn, O and adventitious C. No contaminants from the synthesis process were observed. The ZnO nanowires displayed a doublet at 1021.4 eV and 1044.5 eV [29,30], which correspond to Zn-2p3/2 and Zn-2p1/2 core levels respectively. Furthermore, Fig. 5(b) shows the superimposed C-1s spectrum of modified- and unmodified-ZnO nanowires. The spectral increase in the intensity of the adventitious carbon indicates surface modification of the nanowires with 1-pyrenebutyric acid containing high concentrations of carbon. Furthermore, the slight shift in the binding energy of the carbon peak in the modified ZnO nanowires can be attributed to the bound nature of carbon in 1-pyrenebutyric acid. To ensure covalent functionalization and deduce mode of binding, complementary FTIR-ATR measurements were obtained. The FTIR-ATR spectra of pristine ZnO nanowires, pure PBA and ZnO/PBA nano-heterostructure are illustrated in Fig. 6. As shown in Fig. 6A, pristine ZnO nanowires show no prominent peak in the spectral zone. PBA, on the other hand in Fig. 6B, shows numerous peaks in the fingerprint region, which is typical of aromatic compounds. Peak at 3037 cm−1 correspond to C H stretching of the aromatic pyrene rings. The peaks at 2950 cm−1 and 2873 cm−1 are assigned to C H stretching of CH2 groups [31]. Broad peak in the region of 2500–3300 cm−1 can be attributed to terminal carboxylic groups. The most significant peak centered around 1693 cm−1 is
Fig. 5. (a) XPS survey spectrum of pristine ZnO nanowires, (b) significant increase in adventitious C 1s peak indicating surface modification of nanowires by 1pyrenebutyric acid.
Fig. 6. FTIR-ATR spectrum of (A) pristine ZnO nanowires, (B) pure PBA and, (C) ZnO nanowire–PBA heterostructure.
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4. Conclusions High quality ZnO nanowires, synthesized through an atmospheric chemical vapor deposition process and appropriately characterized, were covalently functionalized with 1-pyrenebutyric acid (PBA) fluorophore. The covalent functionalization of PBA onto ZnO nanowires was determined through XPS and FT-IR ATR spectroscopic techniques. XPS analysis demonstrated an increase in adventitious peak intensity post-functionalization, which is indicative of the presence of PBA on nanowire surface. Complementary evidence for determining nature of bonding was sought through FT-IR ATR. Through fluorescence quenching of PBA by p-nitrophenol, a detection limit up to 28 ppb was envisaged. It was therefore proposed that ZnO nanowires functionalized with PBA receptor represent a novel hybrid system, which could be used to develop ultra-sensitive detection platforms for biosensing applications. Fig. 7. Photoluminescence spectrum showing fluorescence quenching of 1pyrenebutyric acid with p-nitrophenol (inset: UV–vis spectrum of 1-pyrenebutyric acid).
due to carbonyl (C O) moiety of the free terminal carboxylic acid group, while weak peak at 1274 cm−1 is believed to be from C O stretching vibrations. The bound nature of PBA onto ZnO nanowire surface is clearly indicated by absence of C O peak at 1693 cm−1 in Fig. 6C. This clearly suggests bidentate binding mode of PBA on ZnO. Furthermore, typical C H vibration peaks in Fig. 6C, also indicate covalently bound PBA rather than unbound and physisorbed PBA molecules. Fig. 7 shows the fluorescence quenching occurring as a result of the interaction between 1-pyrenebutyric acid (1 M) and pnitrophenol (0.5 M) in solution. Additionally, the inset of Fig. 7 shows the absorption spectrum of PBA. Based on the extent of the fluorescence quenching, a detection limit on the order of 28 ppb was determined. Collisional quenching is generally accepted as the dominant de-excitation pathway, where energy transfer from *-orbitals of pyrene groups to the -orbitals of p-nitrophenol results in the quenching of fluorescence. This enables extremely sensitive detection that could enable the ultratrace detection of p-nitrophenol in biological systems. Covalently functionalizing 1-pyrenebutyric acid on ZnO nanowire, however, could lead to localized and closely spaced pyrene groups, which could result in an intense but structureless broad peak centered at ca. 470 nm attributed to excimer formation. These excimeric peaks could also be utilized for sensing nitroaromatics, including p-nitrophenol, as suggested by Bai et al. [32]. These findings illustrate that the optical effect of 1-pyerenebutyric acid on ZnO nanowires can be modulated, which in turn could result in the development of an extremely sensitive biosensor for p-nitrophenol detection. At this juncture, it is only rational to consider the reversibility of this hybrid probe system. Since our intent is to use this system for sensing vapors of p-nitrophenol, we believe that this issue can be addressed more appropriately at device level. We are currently investigating the possibility of reversing the sensing cycle through dissociation of charge-transfer complex, formed between receptor and analyte, through electron injection into the device. Furthermore, ZnO nanowire arrays also provide a robust support structure for grafting the receptor while concomitantly being beneficial for device fabrication due to cost-effective synthesis methods. In addition, it might provide the required conductive pathways to develop a synergistic opto-electronic platform. Therefore, we believe that covalently functionalized 1-pyrenebutyric acid on ZnO nanowires could potentially be an extremely sensitive platform for p-nitrophenol biosensor fabrication.
Acknowledgement This work was partially funded by the U.S. Army Contract No. W31P4Q-09-D-0028, AMRDEC in Huntsville, AL. References [1] R.C. Gupta, Toxicology of Organophosphate and Carbamate Compounds, Elsevier Academic Press, Burlington, MA, 2005. [2] R. Stephens, A. Spurgeon, I.A. Calvert, J. Beach, L.S. Levy, et al., Lancet 345 (1995) 1135–1139. [3] R.E.D. Fact Sheet, Paranitrophenol (1998). [4] P. Mulchandani, C.M. Hangarter, Y. Lei, W. Chen, A. Mulchandani, Biosensors and Bioelectronics 21 (2005) 523–527. [5] A. Uzer, E. Ercag, R. Apak, Analytica Chimica Acta 505 (2004) 83–93. [6] A. Almasi, E. Fischer, P. Perjesi, Journal of Biochemical and Biophysical Methods 69 (2006) 43–50. [7] N. Masque, R.M. Marce, F. Borull, P.A.G. Cormack, D.C. Sherrington, Analytical Chemistry 72 (2000) 4122–4126. [8] R.H. Yang, K.M. Wang, L.P. Long, W.H. Chan, X.H. Yang, Analyst 127 (2002) 119–124. [9] D. Patra, A.K. Misra, Sensors and Actuators B: Chemical 80 (2001) 278–282. [10] X. Yang, G.L. Shen, R.Q. Yu, Microchimica Acta 136 (2001) 73–78. [11] S. Paliwal, M. Wales, T. Good, J. Grimsley, J. Wild, et al., Analytica Chimica Acta 596 (2007) 9–15. [12] J. Watt, S. Cheong, M.F. Toney, B. Ingham, J. Cookson, et al., ACS Nano 4 (2010) 396–402. [13] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W.V. Schalkwijk, Nature Materials 4 (2005) 366–377. [14] N. Kumar, A. Dorfman, J.I. Hahm, Nanotechnology 17 (2006) 2875–2881. [15] B.V. Kumar, H.S.B. Naik, D. Girija, B. Kumar, Journal of Chemical Sciences 123 (2011) 615–621. [16] H.E. Unalan, P. Hiralal, D. Kuo, B. Parekh, G. Amaratunga, et al., Journal of Materials Chemistry 18 (2008) 5909–5912. [17] O. Lupan, S. Shishiyanu, L. Chow, T. Shishiyanu, Thin Solid Films 516 (2008) 3338–3345. [18] C.Y. Lee, T.Y. Tseng, S.Y. Li, P. Lin, Tamkang Journal of Science and Engineering 6 (2003) 127–132. [19] L. SelegArd, V. Khranovskyy, F. Soderlind, C. Vahlberg, M. Ahren, P.O. Call, R. Yakimova, K. Udval, ACS Applied Materials and Interfaces 2 (2010) 2128–2135. [20] S.A. Ansari, Q. Husain, S. Qayyum, A. Azam, Food and Chemical Toxicology 49 (2011) 2107–2115. [21] A. Choi, K. Kim, H.I. Jung, S.Y. Lee, Sensors and Actuators B: Chemical 148 (2010) 577–582. [22] R.S. Wagner, W.C. Ellis, Applied Physics Letters 6 (1964) 89–90. [23] O. Taratula, E. Galoppini, D. Wang, D. Chu, Z. Zhang, H. Chen, G. Saraf, Y. Lu, Journal of Physical Chemistry B 110 (2006) 6506–6515. [24] A. Gupta, B.C. Kim, C.C. Watkins, S.C. Street, E. Edwards, C. Brantley, P. Ruffin, Journal of Nanotechnology in Engineering and Medicine 2 (2011) 011010–11013. [25] Z. Zhang, S.J. Wang, T. Yu, T. Wu, Journal of Physical Chemistry C 111 (2007) 17500–17505. [26] F.K. Shan, G.X. Liu, W.J. Lee, G.H. Lee, I.S. Kim, B.C. Shin, Applied Physics Letters 86 (2005) 221910–221913. [27] P. Lianos, G. Cremel, Photochemistry and Photobiology 31 (1979) 429–434. [28] W. Chen, N.B. Zuckerman, J.P. Konopelski, S. Chen, Analytical Chemistry 82 (2010) 461–465. [29] NIST X-ray Photoelectron Spectroscopy Database, version 3.5, http://srdata.nist.gov/xps/. [30] B.R. Strohmeier, D.M. Hercules, Journal of Catalysis 86 (1984) 266–279.
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[31] B. Smith, Infrared Spectral Interpretation: A Systematic Approach, CRC Press, Boca Raton, FL, 1999. [32] H. Bai, C. Li, G. Shi, ChemPhysChem 9 (2008) 1908–1913. Anurag Gupta received his B.S. degree from Indian Institute of Technology-Kanpur, India and is currently a senior Ph.D. student in the Tri-Campus Materials Science program at The University of Alabama-Tuscaloosa. His current research interests are in the area of development of nanosensors for military applications, surface chemistry of nanostructures and surface-physics. He has been the recipient of Spain-Hickman scholarship for best international graduate student at The University of Alabama. Furthermore, he has been awarded the prestigious graduate council research and creative activity fellowship for 2012–2013 to complete his
dissertation research. He is a student member of MRS, IEEE and IMAPS professional societies. Bruce C. Kim received his Ph.D. degree in Electrical and Computer Engineering from Georgia Institute of Technology (Georgia Tech), M.S. from the University of Arizona and B.S.E.E. degree from the University of California, Irvine (UCI). His current research interests are in nanotechnology for biomedicine and neural sensors, synthesis of nanowires, testability for RFIC and SoC, sensor characterizations, VLSI circuits and Nano Packaging. Some of his research projects have been supported by NSF CAREER, DARPA, US Army and semiconductor industry. Dr. Kim is life member and fellow of IMAPS. He is a senior member of IEEE and BoG member of the IEEE CPMT Society.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Covalent functionalization of zinc oxide nanowires for high sensitivity p-nitrophenol detection in biological systems Anurag Gupta a,∗ , Bruce C. Kim a , Eugene Edwards b , Christina Brantley b , Paul Ruffin b a b
The University of Alabama, 101 Houser Hall, Tuscaloosa, AL 35487, USA U.S. Army, RDECOM/AMRDEC, 5400 Fowler Road, Redstone Arsenal, AL 35898, USA
a r t i c l e
i n f o
Article history: Received 15 March 2012 Received in revised form 14 June 2012 Accepted 13 August 2012 Available online 5 September 2012 Keywords: Zinc oxide nanowire Surface functionalization 1-Pyrenebutyric acid Fluorescent sensing
a b s t r a c t High-quality zinc oxide (ZnO) nanowires were synthesized using the atmospheric chemical vapor deposition technique and were appropriately characterized. Subsequently, the nanowire surface was covalently grafted with 1-pyrenebutyric acid (PBA) fluorophore, and surface-sensitive X-ray photoelectron spectroscopy and Fourier transform infrared-attenuated total reflectance spectroscopy were utilized to confirm the functionalization of 1-pyrenebutyric acid on the nanowire surface. Additionally, photoluminescence (PL) measurements were used to evaluate the optical behavior of pristine nanowires. Through fluorescence quenching of 1-pyrenebutyric acid by p-nitrophenol, a detection limit of 28 ppb was estimated. Based on these findings, ZnO nanowires functionalized with 1-pyrenebutyric acid are envisaged as extremely sensitive platforms for the ultra-trace detection of p-nitrophenol in biological systems. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nitrophenols are a class of organic compounds generated as degradation products of many organophosphorus compounds (OPs). OPs are key components of modern agricultural products such as herbicides and insecticides. Although these compounds do not accumulate in biological systems, they exhibit high toxicity and are purported to have carcinogenic effects on human subjects [1]. Empirical evidence suggests that these OPs have neurotoxic effects in that they inhibit enzymatic activity related to nerve transmission [2]. Furthermore, p-nitrophenol has been found to be an environmental contaminant in surface water and can be corrosive and acutely toxic when ingested orally, resulting in headaches, nausea and cyanosis [3,4]. Therefore, detection of p-nitrophenol is a major biological and environmental concern, and it is crucial to find simple and straightforward methodologies for ultratrace detection. Previous investigations have developed numerous detection methodologies for trace sensing of p-nitrophenols. Conventional methods include UV–vis spectrophotometric methods [5], high performance liquid chromatography (HPLC) [6], molecular imprinted polymer solid phase extraction [7] and optical membrane based sensors [8–10]. Although all these methods are capable of sensitive detection, they are complicated and thus have limited potential for large-scale sensor development due to cost and
∗ Corresponding author. Tel.: +1 205 764 3925. E-mail address: [email protected] (A. Gupta). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.08.002
other practical limitations. Hence, an approach that ensures costeffective ultratrace detection remains to be found. Fluorescence-based methods offer a prospective solution in terms of detection capabilities and have been shown to detect ppb levels of analyte. Fluorophores have enabled researchers to study biological phenomenon in living tissues and cells. When used in isolation or appropriately tagged with receptors, they can detect specific binding events with molecular-level precision, providing fundamental information. However, the use of fluorescent probes can be extremely limited for several reasons. First, the toxicity of fluorophore to biological systems severely limits the choices available for a particular application. Consequently, identifying a fluorophore that can sense a particular analyte within a ppb level detection limit and is biologically compatible is an extremely challenging task. For example, Paliwal et al. have reported a coumarin-based fluorescent p-nitrophenol sensor with a detection limit of 128 M, which is insufficient for nanomolar- or picomolar-level detection [11]. Secondly, it is likely that the identified fluorophore may not offer flexibility in detecting a spectrum of compounds and may be limited to a particular analyte. This could be a bottleneck to large-scale biosensor fabrication, where robust biosensing platforms capable of sensing multiple compounds are required. Finally, the fluorophore required must bind reversibly to the analyte. This ensures that the fabricated biosensor can be used for multiple cycles and be refreshed if needed. The aforementioned challenges can be bypassed through the design of novel sensing platforms and the utilization of identified fluorophores for enhanced signal-to-noise by functionalizing them onto appropriate, biologically compatible substrates. This can significantly
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increase the sensitivity of a fluorophore while offering flexibility in terms of sensor design. The development of high surface area nanostructures is currently an area of extensive research, particularly with respect to applications in the fields of catalysis, gas storage and sensor technology [12,13]. Organic functionalized nanowire arrays are therefore an excellent choice for biosensing platforms as their large surface area can potentially aid in an extremely sensitive detection of analyte. This surface-related phenomenon coupled with quantum confinement effects, results in excellent optical and electrical properties that can be tuned to address specific needs. Furthermore, the porous structure of these nanowires facilitates the rapid diffusion of analyte molecules to binding sites, leading to a faster response rate in the sensor. Zinc oxide (ZnO) nanowires, in addition to demonstrating aforementioned advantages, can be grown cost-effectively over an insulating substrate. This facilitates device fabrication as orientation of ZnO nanowires can be easily controlled to yield conductive mesh-like structure over the substrate itself, thereby reducing processing costs. Furthermore, ease of tuning the optical properties through precise control of nanowire diameter might provide additional advantages in terms of a hybrid opto-electronic platform for actual biosensor fabrication. Accordingly, this study involves the use of ZnO nanowires, which have previously shown very good potential in terms of aiding optical detection of biomolecules [14] along with other applications related to nanocatalysis, photovoltaics, OLED-based displays and chemical gas sensors [15–18]. Furthermore, functionalized ZnO nanowires have been demonstrated as excellent sensing platforms for, both chemical and biological sensing. SelegArd et al. reported two-step functionalization process for ZnO nanoparticle functionalization [19]. High affinity for biotin was reported using the functionalized nanostructures with potential applications in biosensing and biological imaging. Similarly, Ansari et al. [20] along with Choi et al. [21] reported biosensing platforms using functionalized ZnO nanowires. In this work we demonstrate the synthesis of a ZnO nanowire heterostructure, that has been covalently functionalized with pyrenebutyric acid (PBA) for prospective trace p-nitrophenol detection in biological systems. The ZnO nanowires used for functionalization are synthesized through a simple and straightforward atmospheric chemical vapor deposition (ACVD) process. Appropriate characterization studies along with evidence to support the hypothesis of ultrasensitive detection using PBA are also presented. We then outline the possible mechanism of sensing through these functionalized nano-heterostructures.
2. Materials and methods 2.1. Synthesis of ZnO nanowires ZnO nanowires were synthesized on silicon (1 0 0) substrate (MTI-Corp.) using vapor–liquid–solid technique. Briefly, ZnO (99.9%, J T Baker) powder and graphite powder (99.99%, 300 mesh, Alfa Aesar) were mixed in equal proportion and loaded onto a silica boat to serve as the precursor vapor source for the chemical vapor deposition process. The boat loaded with this powder mixture was introduced in a customized CVD furnace operating at atmospheric pressure. Silicon (1 0 0) substrates, ultrasonically cleaned for 30 min to remove contaminants from the surface, were subsequently coated with a thin layer of gold (ca. 4 nm) to serve as a catalyst during the growth process and were kept downstream in the furnace. Carrier gas, a mixture of argon and oxygen, was introduced from one end of the furnace while the temperature of the furnace was raised to 950 ◦ C. At this high temperature, the gold layer transformed to gold nanodots, which served as nucleation
sites for ZnO nanowire growth. The zinc vapors generated by the reduction of ZnO were carried to the substrate by the carrier gas, where the supersaturation of gold nanodots on the silicon (1 0 0) led to the growth of ZnO nanowire through the vapor–liquid–solid (VLS) mechanism [22]. A growth time of 30 min was allowed. Subsequently, the substrates were removed after furnace cooling and were observed to be covered with a gray coating. 2.2. Functionalization of ZnO nanowires The substrates coated with pristine nanowires were dipped in a 10−3 M solution of the fluorophore, 1-pyrenebutyric acid (VWR corp.), prepared in acetone and subjected to mild agitation with a common laboratory shaker for 1 h. Thereafter, the samples were immersed in a fresh solution of 1-pyrenebutyric acid in acetone, of the same concentration and were re-agitated for 1 h. This process was repeated three times to ensure uniform and complete coating of the nanowires with 1-pyrenebutyric acid. Subsequently, the samples were copiously rinsed for 5 min in pure solvent to remove physisorbed or free fluorophore, were blow-dried gently with nitrogen and were stored in a dry box for 1 week to ensure complete solvent evaporation. It is to be noted that functionalization of ZnO nanowires with carboxylic acid moiety has been demonstrated in previous reports [23,24]. Long chain organic acids form stable covalent bonds on nanowire surface instead of chemically damaging the surface. 2.3. Characterization and fluorescence studies The morphology and composition of the synthesized ZnO nanowires were characterized using field emission-scanning electron microscopy (FE-SEM, JEOL 7000) and energy dispersive X-ray analysis (EDAX, Oxford Instruments). For analyzing crystal structure transmission electron microscopy (TEM, FEI-Technai) equipped with a LaB6 source operated at 200 kV and X-ray diffraction (XRD, Bruker AXS) with Cu-K␣ radiation was used. The surface modification of the pristine nanowires was probed using the surface-sensitive technique of X-ray photoelectron spectroscopy (XPS, Kratos Axis 165 XPS/Auger). XPS is highly efficient in providing surface information and quantification, and it has depth resolution of sub-nm in terms of information it can provide from the surface. The survey scan was performed at a pass energy of 160 eV; for high-resolution scans, a pass energy of 40 eV was utilized. To ensure covalent bonding, complementary Fourier transform infrared-attenuated total reflectance (FTIR-ATR) measurements were obtained from PerkinElmer Spectra 100 equipped with diamond crystal tip. UV–vis measurements were taken on a Cary 50 spectrometer while the fluorescence data were obtained on a FluoroMax-3 (Jobin Yvon) spectrophotometer equipped with a 150 W Xe lamp. A standard 1 cm quartz cuvette was employed to ensure transparency in the UV region of the spectrum. 3. Results and discussion The morphology of the pristine ZnO nanowires was confirmed through FE-SEM investigations as shown in Fig. 1(a) and (b). The synthesized nanowires were 60–80 nm in diameter and 3–5 m in length. We also observed that the synthesized nanowires were randomly oriented and lacked any vertical alignment with the substrate. This is believed to be the result of high lattice mismatch between the silicon substrate and ZnO. Furthermore, Fig. 1(b) clearly shows that in addition to high aspect ratio growth, small percentages of the ZnO nanowires exhibit lateral growth. This can be attributed to a high supersaturation of zinc vapor in the tube furnace or to the depletion of catalyst from growth fronts [25]. We believe that in our case the latter effect played a dominant role
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Fig. 2. X-ray diffraction spectrum of pristine ZnO nanowires. Fig. 1. (a), (b) FE-SEM micrographs of as-synthesized ZnO nanowires, (c) EDAX spectrum of assynthesized.
as gold catalyst nanoparticles were absent from the tips of the nanowires. Through EDAX analysis, as shown in Fig. 1(c), it was determined that no contaminant phases were present in the synthesized nanowires. Furthermore, the atomic percentages of zinc and oxygen were found to be in agreement with the stoichiometric ratio of the molecule, which indicated that ZnO was the only constituent of the synthesized nanowires. The orientational preference of the synthesized nanowires was determined through XRD investigations as shown in Fig. 2. It can be seen that nanowires tend to be oriented along (1 0 1) plane, with a significant proportion oriented along (0 0 2) plane. This significant proportion is indicative of wurtzite phase of ZnO nanowires. To confirm hexagonal wurtzite phase and single crystalline nature of ZnO nanowires, TEM analysis was accordingly performed. Fig. 3(a) shows a typical low magnification image of a ZnO nanowire, which is completely consistent with FE-SEM investigations. The inset of Fig. 3(a) shows the selected area electron diffraction (SAED) pattern, which confirms the high crystallinity of
the synthesized nanowires along with a wurtzite-type structure. Furthermore, Fig. 3(b) presents the HR-TEM (high resolution-TEM) image of the pristine nanowire, which conforms with empirical observations of ZnO nanowires’ tendency toward c-axis growth direction via a vapor–liquid–solid process and typical interplanar spacing of 0.52 nm. To determine the optical characteristics and estimate the defect concentration of the pristine nanowires, photoluminescence (PL) was measured. The resulting PL curve is shown in Fig. 4. The spectrum was found to be clearly dominated by the emission peak at 380 nm (termed as NBE or near band-edge emission peak), originating from the radiative recombination of donor-bound excitons. Additionally, the defect-related peak centered around 500 nm (termed as DL or deep-level emission peak) is extremely weak. Although the exact origin of this deep-level emission is still unclear, it is, however, generally accepted that it potentially stems from oxygen-related vacancy defects [26]. Since in our case this defectrelated peak is almost non-existent, it can be safely deduced that the synthesized nanowires are practically defect free. This observation is particularly important as high-quality, defect-free
Fig. 3. (a) Low resolution-TEM image of a single ZnO nanowire (inset: SAED pattern from the nanowire), (b) high resolution-TEM image showing lattice spacing in growth direction.
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Fig. 4. Photoluminescence spectrum of pristine ZnO nanowires showing band-edge emission.
nanowires can potentially aid in the enhancement of optical response by suppressing the charge transfer to defect sites, thereby affecting the quantum yield of the fluorophore while simultaneously increasing the sensitivity. As mentioned previously, we used 1-pyrenebutyric acid as the fluorophore because it can be grafted onto the ZnO nanowire surface and could potentially provide ultrasensitive detection capabilities for p-nitrophenol present in biological systems. This choice was also based on Lianos and Cremel’s [27] finding that pyrenebased compounds can be used for probing biological structures. Additionally, these compounds’ nitroaromatic sensing capabilities on ruthenium nanoparticles have been independently evaluated by Chen et al. [28]. To determine that the pristine ZnO nanowires had been covalently functionalized by 1-pyrenebutyric acid, we utilized the XPS technique. XPS is a highly surface-sensitive technique that yields spectral information from the surface with sub-nm resolution, thereby making it an appropriate method to determine the surface functionalization of the pristine nanowires. The survey scan from the pristine nanowire sample shown in Fig. 5(a) indicates the presence of the following elements: Zn, O and adventitious C. No contaminants from the synthesis process were observed. The ZnO nanowires displayed a doublet at 1021.4 eV and 1044.5 eV [29,30], which correspond to Zn-2p3/2 and Zn-2p1/2 core levels respectively. Furthermore, Fig. 5(b) shows the superimposed C-1s spectrum of modified- and unmodified-ZnO nanowires. The spectral increase in the intensity of the adventitious carbon indicates surface modification of the nanowires with 1-pyrenebutyric acid containing high concentrations of carbon. Furthermore, the slight shift in the binding energy of the carbon peak in the modified ZnO nanowires can be attributed to the bound nature of carbon in 1-pyrenebutyric acid. To ensure covalent functionalization and deduce mode of binding, complementary FTIR-ATR measurements were obtained. The FTIR-ATR spectra of pristine ZnO nanowires, pure PBA and ZnO/PBA nano-heterostructure are illustrated in Fig. 6. As shown in Fig. 6A, pristine ZnO nanowires show no prominent peak in the spectral zone. PBA, on the other hand in Fig. 6B, shows numerous peaks in the fingerprint region, which is typical of aromatic compounds. Peak at 3037 cm−1 correspond to C H stretching of the aromatic pyrene rings. The peaks at 2950 cm−1 and 2873 cm−1 are assigned to C H stretching of CH2 groups [31]. Broad peak in the region of 2500–3300 cm−1 can be attributed to terminal carboxylic groups. The most significant peak centered around 1693 cm−1 is
Fig. 5. (a) XPS survey spectrum of pristine ZnO nanowires, (b) significant increase in adventitious C 1s peak indicating surface modification of nanowires by 1pyrenebutyric acid.
Fig. 6. FTIR-ATR spectrum of (A) pristine ZnO nanowires, (B) pure PBA and, (C) ZnO nanowire–PBA heterostructure.
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4. Conclusions High quality ZnO nanowires, synthesized through an atmospheric chemical vapor deposition process and appropriately characterized, were covalently functionalized with 1-pyrenebutyric acid (PBA) fluorophore. The covalent functionalization of PBA onto ZnO nanowires was determined through XPS and FT-IR ATR spectroscopic techniques. XPS analysis demonstrated an increase in adventitious peak intensity post-functionalization, which is indicative of the presence of PBA on nanowire surface. Complementary evidence for determining nature of bonding was sought through FT-IR ATR. Through fluorescence quenching of PBA by p-nitrophenol, a detection limit up to 28 ppb was envisaged. It was therefore proposed that ZnO nanowires functionalized with PBA receptor represent a novel hybrid system, which could be used to develop ultra-sensitive detection platforms for biosensing applications. Fig. 7. Photoluminescence spectrum showing fluorescence quenching of 1pyrenebutyric acid with p-nitrophenol (inset: UV–vis spectrum of 1-pyrenebutyric acid).
due to carbonyl (C O) moiety of the free terminal carboxylic acid group, while weak peak at 1274 cm−1 is believed to be from C O stretching vibrations. The bound nature of PBA onto ZnO nanowire surface is clearly indicated by absence of C O peak at 1693 cm−1 in Fig. 6C. This clearly suggests bidentate binding mode of PBA on ZnO. Furthermore, typical C H vibration peaks in Fig. 6C, also indicate covalently bound PBA rather than unbound and physisorbed PBA molecules. Fig. 7 shows the fluorescence quenching occurring as a result of the interaction between 1-pyrenebutyric acid (1 M) and pnitrophenol (0.5 M) in solution. Additionally, the inset of Fig. 7 shows the absorption spectrum of PBA. Based on the extent of the fluorescence quenching, a detection limit on the order of 28 ppb was determined. Collisional quenching is generally accepted as the dominant de-excitation pathway, where energy transfer from *-orbitals of pyrene groups to the -orbitals of p-nitrophenol results in the quenching of fluorescence. This enables extremely sensitive detection that could enable the ultratrace detection of p-nitrophenol in biological systems. Covalently functionalizing 1-pyrenebutyric acid on ZnO nanowire, however, could lead to localized and closely spaced pyrene groups, which could result in an intense but structureless broad peak centered at ca. 470 nm attributed to excimer formation. These excimeric peaks could also be utilized for sensing nitroaromatics, including p-nitrophenol, as suggested by Bai et al. [32]. These findings illustrate that the optical effect of 1-pyerenebutyric acid on ZnO nanowires can be modulated, which in turn could result in the development of an extremely sensitive biosensor for p-nitrophenol detection. At this juncture, it is only rational to consider the reversibility of this hybrid probe system. Since our intent is to use this system for sensing vapors of p-nitrophenol, we believe that this issue can be addressed more appropriately at device level. We are currently investigating the possibility of reversing the sensing cycle through dissociation of charge-transfer complex, formed between receptor and analyte, through electron injection into the device. Furthermore, ZnO nanowire arrays also provide a robust support structure for grafting the receptor while concomitantly being beneficial for device fabrication due to cost-effective synthesis methods. In addition, it might provide the required conductive pathways to develop a synergistic opto-electronic platform. Therefore, we believe that covalently functionalized 1-pyrenebutyric acid on ZnO nanowires could potentially be an extremely sensitive platform for p-nitrophenol biosensor fabrication.
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[31] B. Smith, Infrared Spectral Interpretation: A Systematic Approach, CRC Press, Boca Raton, FL, 1999. [32] H. Bai, C. Li, G. Shi, ChemPhysChem 9 (2008) 1908–1913. Anurag Gupta received his B.S. degree from Indian Institute of Technology-Kanpur, India and is currently a senior Ph.D. student in the Tri-Campus Materials Science program at The University of Alabama-Tuscaloosa. His current research interests are in the area of development of nanosensors for military applications, surface chemistry of nanostructures and surface-physics. He has been the recipient of Spain-Hickman scholarship for best international graduate student at The University of Alabama. Furthermore, he has been awarded the prestigious graduate council research and creative activity fellowship for 2012–2013 to complete his
dissertation research. He is a student member of MRS, IEEE and IMAPS professional societies. Bruce C. Kim received his Ph.D. degree in Electrical and Computer Engineering from Georgia Institute of Technology (Georgia Tech), M.S. from the University of Arizona and B.S.E.E. degree from the University of California, Irvine (UCI). His current research interests are in nanotechnology for biomedicine and neural sensors, synthesis of nanowires, testability for RFIC and SoC, sensor characterizations, VLSI circuits and Nano Packaging. Some of his research projects have been supported by NSF CAREER, DARPA, US Army and semiconductor industry. Dr. Kim is life member and fellow of IMAPS. He is a senior member of IEEE and BoG member of the IEEE CPMT Society.