Journal of Immunoassay and Immunochemistry, 36:142–148, 2015 Copyright © Taylor & Francis Group, LLC ISSN: 1532-1819 print/1532-4230 online DOI: 10.1080/15321819.2014.908129
RESISTANCE-BASED BIOSENSOR OF MULTI-WALLED CARBON NANOTUBES
E. S. Kolosovas-Machuca,1 G. Vera-Reveles,1,2 M. C. Rodríguez-Aranda,3 L. C. Ortiz-Dosal,1 Emmanuel Segura-Cardenas,4 and Francisco J. Gonzalez1 1 Universidad Autónoma de San Luis Potosí, Coordinación para la Innovación y la Aplicación de la Ciencia y la Tecnología, San Luis Potosí, México 2 Departamento de Ciencias Básicas, Instituto Tecnológico de San Luis Potosí, Soledad de Graciano Sánchez, México 3 Centro de Investigación y de Estudios Avanzados del I.P.N. Unidad Querétaro, Querétaro, México 4 Instituto de Investigación en Comunicación Óptica, Universidad Autónoma de San Luis Potosí. San Luis Potosí, México
Multi-Walled Carbon Nanotubes (MWNTs) are a good choice for resistive biosensors due to their great resistance changes when immunoreactions take place, they are also low-cost, more biocompatible than single-walled carbon nanotubes, and resistive measurement equipment is usually not expensive and readily available. In this work a novel resistive biosensor based on the immobilization of an antigen through a silanization process over the surface of Multi-Walled Carbon Nanotubes (MWNTs) is reported. Results show that the biosensor increases its conductivity when adding the antigen and decreases when adding the antibody making them good candidates for disease diagnosis. Keywords biosensor, multi-walled carbon nanotubes, resistive sensor
INTRODUCTION The early detection of pathogens plays a crucial role in the prevention of disease spread. The most sensitive and specific assay diagnosis for infectious disease are the laboratory-based polymerase chain reaction (PCR) or Enzyme-Linked ImmunoSorbent Assay (ELISA) methods.[1,2] However, the methods mentioned above are costly, time consuming, and Address correspondence to E. S. Kolosovas-Machuca, Universidad Autónoma de San Luis Potosí, Coordinación para la Innovación y la Aplicación de la Ciencia y la Tecnología, San Luis Potosí, México. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline. com/ljii.
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need qualified personnel and sophisticated instrumentation. Thus, alternative approaches are desirable. Immunosensors constitute an alternative and promising immunoassay technique that can be widely applied in many different areas.[3,4] The rapid development of new nanomaterials and nanotechnologies provides new opportunities in the biosensensing area; recently, the interest in using carbon nanotubes (CNTs) for biosensing applications is growing. CNTs have been widely used in this field because of their high surface area and surface/volume ratio, good electrical conductivity, and significant mechanical strength.[5−8] The possible integration of sensors in daily materials such as yarns, rubber, or paper materials emerged as an attempt to solve the cost-related issues.[9−12] In this article, the properties of multi-walled carbon nanotubes (MWCNTs) for the development of a resistive immunosensor are explored, since they could be a good alternative to the electrochemical immunosensors due to their low cost, readily available instrumentation, and ease of use. The immobilization of biomolecules onto solid substrates is of crucial importance for all immunosensors. The method used in this work is based on the creation of a covalent linkage between the MWNTs and a specific protein. The links are created following three main steps: (1) oxidation, utilizing a mixture of both concentrated sulfuric and nitric acid; (2) silanization, using 3-aminopropyltriethoxysilane (APTES), this is an effective method for the modification of the physical and chemical properties of the treated material, the organofunctional silane coupling agents allow us to chemically join two materials, and finally; (3) modification of the last layer, using glutaraldehyde as a fixative. The functionalization generates functional groups at the surface of the MWNTs which then makes possible to link a protein onto the biosensor’s surface. Bovine serum albumin (BSA) is an important modifying material due to the presence of amino groups in its structure. After the functionalization process, BSA and anti-BSA were linked onto the substrates. In this article, a disposable resistance-based immunosensor using functionalized multi-walled carbon nanotubes is proposed. METHOD 3-aminopropyltriethoxylsilane (APTES), glutaraldehyde (50%), phosphate buffered saline (PBS, pH=7.4, 0.01M), acetone, 2-propanol, ethanol, bovine serum albumin (BSA) and anti-bovine serum albumine (anti-BSA) were purchased from Sigma–Aldrich. All aqueous solutions were prepared in deionized water. MWNTs were purified prior to use by a 2 M hydrochloric acid solution. 50 mg of MWNTs were dispersed in 200 mL of concentrated H2 SO4 /HNO3 (3:1 v/v) solution at room temperature for 24 hr. Forty milligrams of MWNTs were dispersed in a freshly prepared ethanolic APTES solution (6%, v/v) for 5 hr at room temperature. The product
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FIGURE 1 Schematic representation of modification process of MWNTs.
was purified by repeated washing with water followed by acetone. The resulting silanized nanotubes (APTES–MWNTs) were separated by filtration and dried under vacuum at 100◦ C for 8 hr. Fourier Transform Infrared (FT-IR) spectra of MWNTs were recorded using a Perkin Elmer FTIR GX System. Scanning electron microscope (SEM) (JEOL JCM-5000) was used to characterize the morphology of the MWNTs. Raman scattering measurements were performed at room temperature using a Jobin-Ybon T64000 spectrometer operating in the triple configuration. Figure 1 shows an outline of the steps followed in the treatment of MWNTs before being deposited on glass: (a) MWNTs without any treatment, (b) purified MWNTs (p-MWNTs), (c) oxidized MWNTs (O-MWNTs), and (d) silanized nanotubes (S-MWNTs). A water solution of MWNTs was deposited on glass substrates of 2.5 x 7.5 cm by means of a syringe and let to dry at room temperature. Two vertical silver stripes (6 mm long, 1.2 mm wide, and 8 mm apart) were deposited atop MWNTs using a conductive silver paint, anchoring copper wires to create electrical contacts on both sides of the film surface to allow the currentvoltage (I-V) characterization. The current voltage (I-V) characteristics were obtained at atmospheric pressure, at room temperature, using a Keithley ® 617 electrometer while varying the applied voltage from 0.05–0.30 V. RESULTS AND DISCUSSION Figure 2 shows scanning electron microscopy (SEM) micrographs of (a) MWNTs and (b) purified MWCNTs (p-MWNTs), a diminution in the CNT’s agglomerates is observed, the diameter range of the p-MWCNTs was 80–100 nm. The Raman spectra shown in Figure 3 were obtained using an excitation line of 514.5 nm. The spectra were collected for pMWNTs and MWNTs. Characteristic Raman active modes were observed for MWNTs at 1354.71cm−1 and at 1368.24cm−1 for p-MWNTs, which corresponds to the disorder induced D band. The tangential G band is observed at 1593.93cm−1 for MWNTs and at 1589.07cm−1 for p-MWNTs. The ratio
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FIGURE 2 SEM image of (a) MWNTs and (b) p-MWNTs.
FIGURE 3 Raman spectra of MWNTs and p-MWNTs obtained from green laser (514.5 nm).
of defects concentration ID/G calculated for MWNTs was 0.98 and 0.42 for p-MWNTs, indicating that the ratio of defects was reduced by 57% after the purification process.[13] FT-IR analysis was performed to determine the chemical changes on the surface of CNTs due to the silane treatment. The FT-IR spectra of the CNTs after the silanization process (S-MWNTs) are presented in Figures 4 and 5. In Figure 4, two characteristic absorption peaks are observed at 1154 and 1079 cm−1 , attributed to a C-O and Si–O vibrations respectively, these peaks do not appear in the MCNTs without the functionalization procedure. In Figure 5, the functional groups hydroxyl, alkyl,
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FIGURE 4 FTIR spectra of MWNTs and S- MWNTs.
FIGURE 5 FTIR spectrum of MWNTs treated with the acid mixture and 3-APTES.
and alkynyl can be observed. The immunosensor was electrically characterized after each functionalization step. We recorded the current vs. voltage (I–V) curves for BSA and anti-BSA immobilization onto the surface. Figure 6 shows the I–V characteristics of the MWNTs, MWNTs + Protein and MWNTs
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FIGURE 6 Experimental I-V values of various samples established over three consecutive runs.
+ Protein + Antigen conditions. A current decrease of about 15.65% was observed at a bias voltage of 0.3 V between the MWNTs + Protein and MWNTs + Protein + Antigen conditions. The resistance of the deposited samples increased as a function of anti- BSA immobilization due to the molecules over the MWNTs work as electron donors contribute with negative charges to the p-type MWNTs in presence of air, which lead to a reduction of the charge.[14] These results are consistent with those obtained by Pozuelo et al.[9] with a paper-based chemiresistor composed of a network of single-wall carbon nanotubes (SWCNTs) and anti-human immunoglobulin G (anti-HIgG). CONCLUSIONS A novel resistive biosensor based on the immobilization of an antigen through a silanization process over the surface of Multi-Walled Carbon Nanotubes (MWNTs) was built and characterized. The characterization procedures show the attachment of silane molecules onto the surface of MWNTs. Also, resistive measurements indicate that the BSA and anti-BSA concentration influences the electrical characteristics of the deposited samples. An increase in conductivity was observed when adding the BSA and a decrease when adding the anti-BSA. These results show that MWNTs have the potential to be used as low-cost immunosensors for disease diagnosis.
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REFERENCES 1. Mackay, I. M. Real-Time PCR in the Microbiology Laboratory. Clin. Microbiol. Infect. 2004, 10(3), 190–212. 2. Wright, P. F.; Nilsson, E.; Van Rooij, E. M.; Lelenta, M.; Jeggo, M. H. Standardisation and Validation of Enzyme-Linked Immunosorbent Assay Techniques for the Detection of Antibody in Infectious Disease Diagnosis. Rev. Sci. Tech. 1993, 12(2), 435–450. 3. Ricci, F.; Volpe, G.; Micheli, L.; Palleschi, G. A Review on Novel Developments and Applications of Immunosensors in Food Analysis. Anal. Chim. Acta 2007, 605(2), 111–129. 4. German, N.; Kausaite-Minkstimiene, A.; Kirlyte, J.; Makaraviciute, A.; Ramanavicius, A.; Mikoliunaite, L.; Ramanaviciene, A. Determination of Antibodies against Human Growth Hormone using a Direct Immunoassay Format and Different Electrochemical Methods. Analyst 2013, 138, 1427–1433. 5. Jacobs, C. B.; Peairs, M. J.; Venton, B. J. Review: Carbon Nanotube Based electrochemical sensors for biomolecules. Anal. Chim. Acta 2010, 662(2), 105–127. 6. Zhong, J.; Song, L.; Meng, J.; Gao, B.; Chu, W.; Xu, H.; Luo, Y.; Guo, J.; Marcelli, A.; Xie, S.; Wu, Z. Bio–nano Interaction of Proteins Adsorbed on Single-Walled Carbon Nanotubes. Carbon NY , 2009, 47 (4), 967–973. 7. Siswana, M.; Ozoemena, K. I.; Nyokong, T.Electrocatalytic Detection of Amitrole on the Multi-Walled Carbon Nanotube – Iron (II) tetra-aminophthalocyanine Platform. Sensors 2008, 8(8) 5096–5105. 8. Ajayan, P. M. Nanotubes from Carbon. Chem. Rev. 1999, 99(7), 1787–1799. 9. Pozuelo, M.; Blondeau, P.; Novell, M.; Andrade, F. J.; Xavier Rius, F.; Riu, J. Paper-based Chemiresistor for Detection of Ultralow Concentrations of Protein. Biosens. Bioelectron. 2013, 49, 462–465. 10. Shim, B. S.; Chen, W.; Doty, C.; Xu, C.; Kotov, N. A. Smart Electronic Yarns and Wearable Fabrics for Human Biomonitoring Made by Carbon Nanotube Coating with Polyelectrolytes. Nano Lett. 2008, 8(12), 4151–4157. 11. Sekitani, T.; Noguchi, Y.; Hata, K.; Fukushima, T.; Aida, T.; Someya, T. A Rubberlike Stretchable Active Matrix using Elastic Conductors. Science 2008, 321(5895), 1468–1472. 12. Wang, L.; Chen, W.; Xu, D.; Shim, B. S.; Zhu, Y.; Sun, F.; Liu, L.; Peng, C.; Jin, Z.; Xu, C.; Kotov, N. A. Simple, Rapid, Sensitive, and Versatile SWNT-Paper Sensor for Environmental Toxin Detection Competitive with ELISA. Nano Lett. 2009, 9(12), 4147–4152. 13. Murphy, H.; Papakonstantinou, P.; Okpalugo, T. I. T. Raman Study of Multiwalled Carbon Nanotubes Functionalized with Oxygen Groups. J. Vac. Sci. Technol. B 2006, 24(2), 715–720. 14. Salehi-Khojin, A.; Khalili-Araghi, F.; Kuroda, M. A.; Lin, K. Y.; Leburton, J.-P.; Masel, R. I. On the Sensing Mechanism in Carbon Nanotube Chemiresistors. ACS Nano 2011, 5(1), 153–158.
Copyright of Journal of Immunoassay & Immunochemistry is the property of Taylor & Francis Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.
RESISTANCE-BASED BIOSENSOR OF MULTI-WALLED CARBON NANOTUBES
E. S. Kolosovas-Machuca,1 G. Vera-Reveles,1,2 M. C. Rodríguez-Aranda,3 L. C. Ortiz-Dosal,1 Emmanuel Segura-Cardenas,4 and Francisco J. Gonzalez1 1 Universidad Autónoma de San Luis Potosí, Coordinación para la Innovación y la Aplicación de la Ciencia y la Tecnología, San Luis Potosí, México 2 Departamento de Ciencias Básicas, Instituto Tecnológico de San Luis Potosí, Soledad de Graciano Sánchez, México 3 Centro de Investigación y de Estudios Avanzados del I.P.N. Unidad Querétaro, Querétaro, México 4 Instituto de Investigación en Comunicación Óptica, Universidad Autónoma de San Luis Potosí. San Luis Potosí, México
Multi-Walled Carbon Nanotubes (MWNTs) are a good choice for resistive biosensors due to their great resistance changes when immunoreactions take place, they are also low-cost, more biocompatible than single-walled carbon nanotubes, and resistive measurement equipment is usually not expensive and readily available. In this work a novel resistive biosensor based on the immobilization of an antigen through a silanization process over the surface of Multi-Walled Carbon Nanotubes (MWNTs) is reported. Results show that the biosensor increases its conductivity when adding the antigen and decreases when adding the antibody making them good candidates for disease diagnosis. Keywords biosensor, multi-walled carbon nanotubes, resistive sensor
INTRODUCTION The early detection of pathogens plays a crucial role in the prevention of disease spread. The most sensitive and specific assay diagnosis for infectious disease are the laboratory-based polymerase chain reaction (PCR) or Enzyme-Linked ImmunoSorbent Assay (ELISA) methods.[1,2] However, the methods mentioned above are costly, time consuming, and Address correspondence to E. S. Kolosovas-Machuca, Universidad Autónoma de San Luis Potosí, Coordinación para la Innovación y la Aplicación de la Ciencia y la Tecnología, San Luis Potosí, México. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline. com/ljii.
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need qualified personnel and sophisticated instrumentation. Thus, alternative approaches are desirable. Immunosensors constitute an alternative and promising immunoassay technique that can be widely applied in many different areas.[3,4] The rapid development of new nanomaterials and nanotechnologies provides new opportunities in the biosensensing area; recently, the interest in using carbon nanotubes (CNTs) for biosensing applications is growing. CNTs have been widely used in this field because of their high surface area and surface/volume ratio, good electrical conductivity, and significant mechanical strength.[5−8] The possible integration of sensors in daily materials such as yarns, rubber, or paper materials emerged as an attempt to solve the cost-related issues.[9−12] In this article, the properties of multi-walled carbon nanotubes (MWCNTs) for the development of a resistive immunosensor are explored, since they could be a good alternative to the electrochemical immunosensors due to their low cost, readily available instrumentation, and ease of use. The immobilization of biomolecules onto solid substrates is of crucial importance for all immunosensors. The method used in this work is based on the creation of a covalent linkage between the MWNTs and a specific protein. The links are created following three main steps: (1) oxidation, utilizing a mixture of both concentrated sulfuric and nitric acid; (2) silanization, using 3-aminopropyltriethoxysilane (APTES), this is an effective method for the modification of the physical and chemical properties of the treated material, the organofunctional silane coupling agents allow us to chemically join two materials, and finally; (3) modification of the last layer, using glutaraldehyde as a fixative. The functionalization generates functional groups at the surface of the MWNTs which then makes possible to link a protein onto the biosensor’s surface. Bovine serum albumin (BSA) is an important modifying material due to the presence of amino groups in its structure. After the functionalization process, BSA and anti-BSA were linked onto the substrates. In this article, a disposable resistance-based immunosensor using functionalized multi-walled carbon nanotubes is proposed. METHOD 3-aminopropyltriethoxylsilane (APTES), glutaraldehyde (50%), phosphate buffered saline (PBS, pH=7.4, 0.01M), acetone, 2-propanol, ethanol, bovine serum albumin (BSA) and anti-bovine serum albumine (anti-BSA) were purchased from Sigma–Aldrich. All aqueous solutions were prepared in deionized water. MWNTs were purified prior to use by a 2 M hydrochloric acid solution. 50 mg of MWNTs were dispersed in 200 mL of concentrated H2 SO4 /HNO3 (3:1 v/v) solution at room temperature for 24 hr. Forty milligrams of MWNTs were dispersed in a freshly prepared ethanolic APTES solution (6%, v/v) for 5 hr at room temperature. The product
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FIGURE 1 Schematic representation of modification process of MWNTs.
was purified by repeated washing with water followed by acetone. The resulting silanized nanotubes (APTES–MWNTs) were separated by filtration and dried under vacuum at 100◦ C for 8 hr. Fourier Transform Infrared (FT-IR) spectra of MWNTs were recorded using a Perkin Elmer FTIR GX System. Scanning electron microscope (SEM) (JEOL JCM-5000) was used to characterize the morphology of the MWNTs. Raman scattering measurements were performed at room temperature using a Jobin-Ybon T64000 spectrometer operating in the triple configuration. Figure 1 shows an outline of the steps followed in the treatment of MWNTs before being deposited on glass: (a) MWNTs without any treatment, (b) purified MWNTs (p-MWNTs), (c) oxidized MWNTs (O-MWNTs), and (d) silanized nanotubes (S-MWNTs). A water solution of MWNTs was deposited on glass substrates of 2.5 x 7.5 cm by means of a syringe and let to dry at room temperature. Two vertical silver stripes (6 mm long, 1.2 mm wide, and 8 mm apart) were deposited atop MWNTs using a conductive silver paint, anchoring copper wires to create electrical contacts on both sides of the film surface to allow the currentvoltage (I-V) characterization. The current voltage (I-V) characteristics were obtained at atmospheric pressure, at room temperature, using a Keithley ® 617 electrometer while varying the applied voltage from 0.05–0.30 V. RESULTS AND DISCUSSION Figure 2 shows scanning electron microscopy (SEM) micrographs of (a) MWNTs and (b) purified MWCNTs (p-MWNTs), a diminution in the CNT’s agglomerates is observed, the diameter range of the p-MWCNTs was 80–100 nm. The Raman spectra shown in Figure 3 were obtained using an excitation line of 514.5 nm. The spectra were collected for pMWNTs and MWNTs. Characteristic Raman active modes were observed for MWNTs at 1354.71cm−1 and at 1368.24cm−1 for p-MWNTs, which corresponds to the disorder induced D band. The tangential G band is observed at 1593.93cm−1 for MWNTs and at 1589.07cm−1 for p-MWNTs. The ratio
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FIGURE 2 SEM image of (a) MWNTs and (b) p-MWNTs.
FIGURE 3 Raman spectra of MWNTs and p-MWNTs obtained from green laser (514.5 nm).
of defects concentration ID/G calculated for MWNTs was 0.98 and 0.42 for p-MWNTs, indicating that the ratio of defects was reduced by 57% after the purification process.[13] FT-IR analysis was performed to determine the chemical changes on the surface of CNTs due to the silane treatment. The FT-IR spectra of the CNTs after the silanization process (S-MWNTs) are presented in Figures 4 and 5. In Figure 4, two characteristic absorption peaks are observed at 1154 and 1079 cm−1 , attributed to a C-O and Si–O vibrations respectively, these peaks do not appear in the MCNTs without the functionalization procedure. In Figure 5, the functional groups hydroxyl, alkyl,
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FIGURE 4 FTIR spectra of MWNTs and S- MWNTs.
FIGURE 5 FTIR spectrum of MWNTs treated with the acid mixture and 3-APTES.
and alkynyl can be observed. The immunosensor was electrically characterized after each functionalization step. We recorded the current vs. voltage (I–V) curves for BSA and anti-BSA immobilization onto the surface. Figure 6 shows the I–V characteristics of the MWNTs, MWNTs + Protein and MWNTs
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FIGURE 6 Experimental I-V values of various samples established over three consecutive runs.
+ Protein + Antigen conditions. A current decrease of about 15.65% was observed at a bias voltage of 0.3 V between the MWNTs + Protein and MWNTs + Protein + Antigen conditions. The resistance of the deposited samples increased as a function of anti- BSA immobilization due to the molecules over the MWNTs work as electron donors contribute with negative charges to the p-type MWNTs in presence of air, which lead to a reduction of the charge.[14] These results are consistent with those obtained by Pozuelo et al.[9] with a paper-based chemiresistor composed of a network of single-wall carbon nanotubes (SWCNTs) and anti-human immunoglobulin G (anti-HIgG). CONCLUSIONS A novel resistive biosensor based on the immobilization of an antigen through a silanization process over the surface of Multi-Walled Carbon Nanotubes (MWNTs) was built and characterized. The characterization procedures show the attachment of silane molecules onto the surface of MWNTs. Also, resistive measurements indicate that the BSA and anti-BSA concentration influences the electrical characteristics of the deposited samples. An increase in conductivity was observed when adding the BSA and a decrease when adding the anti-BSA. These results show that MWNTs have the potential to be used as low-cost immunosensors for disease diagnosis.
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REFERENCES 1. Mackay, I. M. Real-Time PCR in the Microbiology Laboratory. Clin. Microbiol. Infect. 2004, 10(3), 190–212. 2. Wright, P. F.; Nilsson, E.; Van Rooij, E. M.; Lelenta, M.; Jeggo, M. H. Standardisation and Validation of Enzyme-Linked Immunosorbent Assay Techniques for the Detection of Antibody in Infectious Disease Diagnosis. Rev. Sci. Tech. 1993, 12(2), 435–450. 3. Ricci, F.; Volpe, G.; Micheli, L.; Palleschi, G. A Review on Novel Developments and Applications of Immunosensors in Food Analysis. Anal. Chim. Acta 2007, 605(2), 111–129. 4. German, N.; Kausaite-Minkstimiene, A.; Kirlyte, J.; Makaraviciute, A.; Ramanavicius, A.; Mikoliunaite, L.; Ramanaviciene, A. Determination of Antibodies against Human Growth Hormone using a Direct Immunoassay Format and Different Electrochemical Methods. Analyst 2013, 138, 1427–1433. 5. Jacobs, C. B.; Peairs, M. J.; Venton, B. J. Review: Carbon Nanotube Based electrochemical sensors for biomolecules. Anal. Chim. Acta 2010, 662(2), 105–127. 6. Zhong, J.; Song, L.; Meng, J.; Gao, B.; Chu, W.; Xu, H.; Luo, Y.; Guo, J.; Marcelli, A.; Xie, S.; Wu, Z. Bio–nano Interaction of Proteins Adsorbed on Single-Walled Carbon Nanotubes. Carbon NY , 2009, 47 (4), 967–973. 7. Siswana, M.; Ozoemena, K. I.; Nyokong, T.Electrocatalytic Detection of Amitrole on the Multi-Walled Carbon Nanotube – Iron (II) tetra-aminophthalocyanine Platform. Sensors 2008, 8(8) 5096–5105. 8. Ajayan, P. M. Nanotubes from Carbon. Chem. Rev. 1999, 99(7), 1787–1799. 9. Pozuelo, M.; Blondeau, P.; Novell, M.; Andrade, F. J.; Xavier Rius, F.; Riu, J. Paper-based Chemiresistor for Detection of Ultralow Concentrations of Protein. Biosens. Bioelectron. 2013, 49, 462–465. 10. Shim, B. S.; Chen, W.; Doty, C.; Xu, C.; Kotov, N. A. Smart Electronic Yarns and Wearable Fabrics for Human Biomonitoring Made by Carbon Nanotube Coating with Polyelectrolytes. Nano Lett. 2008, 8(12), 4151–4157. 11. Sekitani, T.; Noguchi, Y.; Hata, K.; Fukushima, T.; Aida, T.; Someya, T. A Rubberlike Stretchable Active Matrix using Elastic Conductors. Science 2008, 321(5895), 1468–1472. 12. Wang, L.; Chen, W.; Xu, D.; Shim, B. S.; Zhu, Y.; Sun, F.; Liu, L.; Peng, C.; Jin, Z.; Xu, C.; Kotov, N. A. Simple, Rapid, Sensitive, and Versatile SWNT-Paper Sensor for Environmental Toxin Detection Competitive with ELISA. Nano Lett. 2009, 9(12), 4147–4152. 13. Murphy, H.; Papakonstantinou, P.; Okpalugo, T. I. T. Raman Study of Multiwalled Carbon Nanotubes Functionalized with Oxygen Groups. J. Vac. Sci. Technol. B 2006, 24(2), 715–720. 14. Salehi-Khojin, A.; Khalili-Araghi, F.; Kuroda, M. A.; Lin, K. Y.; Leburton, J.-P.; Masel, R. I. On the Sensing Mechanism in Carbon Nanotube Chemiresistors. ACS Nano 2011, 5(1), 153–158.
Copyright of Journal of Immunoassay & Immunochemistry is the property of Taylor & Francis Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use.
