NANO EXPRESS Open Access Ion-sensing properties of 1D vanadium pentoxide nanostructures Nirton CS Vieira1*, Waldir Avansi2, Alessandra Figueiredo1, Caue Ribeiro3, Valmor R Mastelaro1 and Francisco EG Guimarães1 Abstract The application of one-dimensional (1D) V2O5�nH2O nanostructures as pH sensing material was evaluated. 1D V2O5�nH2O nanostructures were obtained by a hydrothermal method with systematic control of morphology forming different nanostructures: nanoribbons, nanowires and nanorods. Deposited onto Au-covered substrates, 1D V2O5�nH2O nanostructures were employed as gate material in pH sensors based on separative extended gate FET as an alternative to provide FET isolation from the chemical environment. 1D V2O5�nH2O nanostructures showed pH sensitivity around the expected theoretical value. Due to high pH sensing properties, flexibility and low cost, further applications of 1D V2O5�nH2O nanostructures comprise enzyme FET-based biosensors using immobilized enzymes. Keywords: Vanadium pentoxide, Nanostructures, pH sensors, SEGFET, Hydrothermal synthesis Background Proton donor-acceptor property (amphoterism) is charac- teristic of several metal oxides or nitrides. These proper- ties have enabled the development of numerous devices to measure ion activities in chemical environments, including ion-sensitive field-effect transistors (ISFET) [1], capacitive electrolyte-insulator-semiconductors [2], light-addressable potentiometric sensors [3], and separative extended gate field-effect transistors (SEGFET) [4]. All these devices are based on field effect and the surface potential of gate insu- lator material that changes according to the ion concen- tration in the solution, controlling the output signal. ISFET is the most common type of field-effect device used in pH sensors and biosensors because it can be miniatur- ized and manufactured on a large scale. However, in ISFET sensors, the FET is in direct contact with the solu- tion, which can hinder the measurement and immobilization of biomolecules due to their small dimen- sions. As an alternative, a SEGFET [4] or, in a simple way, a sensitive layer connected to the input pin of a high- impedance buffer, such as an operational amplifier [5,6], can be utilized. In both cases, the transduction principle (field effect) is the same. Besides the reuse of the FET in new measurements, the robustness and flexibility of the extended sensitive layer facilitate the processing of new materials to be implemented as ion sensors. Since the technology of field-effect devices is mature, research has focused on the synthesis of new materials to be applied as ion sensitive membranes. Several metal oxi- des or nitrides that have been used as pH sensitive mem- branes have presented the expected response [7-10]. In fact, nanoscale metal oxides can improve the fundamen- tal properties of materials and the performance of devices due to new physical and chemical properties. Recently, one-dimensional (1D) nanostructured materials such as nanowires, nanoribbons and nanotubes have attracted much interest due to their improved properties when compared to similar isotropic nanostructures [11-13]. Vanadium pentoxide (V2O5), which possesses particu- larly interesting physical and chemical properties, has been employed in technological applications as catalytic material [14], in electrochromic devices [15], as battery cathode material [16], and in sensors [17-19]. Several strategies have been developed to obtain 1D V2O5 nanos- tructures. For example, Avansi et al. recently reported an environmentally correct, one-step hydrothermal route for the synthesis of V2O5�nH2O nanostructures with con- trolled morphology and crystalline structure [20]. * Correspondence: nirton@ursa.ifsc.usp.br 1Departamento de Física e Ciências dos Materiais, Instituto de Física de São Carlos, Universidade de São Paulo, Avenida Trabalhador São-carlense 400, São Carlos, São Paulo CP 369/13560-970, Brazil Full list of author information is available at the end of the article © 2012 Vieira et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Vieira et al. Nanoscale Research Letters 2012, 7:310 http://www.nanoscalereslett.com/content/7/1/310 mailto:nirton@ursa.ifsc.usp.br http://creativecommons.org/licenses/by/2.0 Combining SEGFET devices and V2O5�nH2O nanos- tructures, field-effect sensors can be constructed in a simple and low-cost way. In this context of technological applications, we report on the use of 1D V2O5�nH2O nanostructures obtained by a hydrothermal method as pH sensitive membranes in a SEGFET device, which was constructed based on van der Spiegel’s concept [5]. Methods The V2O5�nH2O nanostructures were synthesized by a hydrothermal method which is described in detail else- where [20]. Briefly, this procedure involves dissolving V2O5 micrometric powder (Alfa Aesar, Ward Hill, MA, USA; 99.995% purity) in deionized water, adding hydrogen peroxide (H2O2), and treating the mixture hydrothermally. Different V2O5�nH2O 1D nanostructures were obtained by applying the hydrothermal treatment at different tem- peratures in the same time of synthesis (24 h) [20]. The crystalline phase of the as-obtained samples was investigated by X-ray diffraction (XRD) using a Shi- madzu XRD 6000 diffractometer (Shimadzu Corpor- ation, Nakagyo-ku, Kyoto, Japan) with Cu kα (λ= 1.5406) radiation. The size and morphology of the as-obtained samples were determined using a Zeiss VP Supra 35 field emission scanning transmission electron microscope (FE-STEM; Carl Zeiss AG, Oberkochen, Germany). The as-obtained samples were deposited onto Au- coated substrates by spin coating and connected to the in- put pin of a LF356 JFET operational amplifier, used here as a unity gain buffer. A silver/silver chloride (Ag/AgCl) reference electrode was used to keep the voltage constant. Figure 1 shows a schematic diagram of the SEGFET. Results and discussion The diffractograms in Figure 2 confirm the expected crystalline phase in all the samples under study, i.e., monoclinic phase in the samples synthesized at 160°C and orthorhombic phase in those synthesized at 180°C and 200°C [20]. The bright field scanning transmission electron mi- croscopy (STEM) images shown in Figure 3 confirm the morphology of the resulting nanostructures. As expected, different nanostructures were obtained. The samples synthesized at 160°C show a nanoribbon-like morphology (Figure 3a), while samples synthesized at 180°C and 200°C present, respectively, nanowire-like (Figure 3b) and nanorod-like (Figure 3c) morphologies [20]. SEGFET devices have been used as an alternative to conventional ISFET to isolate FET from analytical chem- ical environments and have presented the same oper- ational characteristics [4,6,9,18]. The robustness and flexibility of the gate in SEGFET devices allow for the Figure 1 Schematic diagram of the SEGFET configuration. The electronic diagram of LF356 operational amplifier is shown. 10 20 30 40 50 60 V 2 O 5 nH 2 O monoclinic V 2 O 5 orthorhombic (c) (b)In te ns ity / a. u. 2 degree (a) Figure 2 XRD diffractograms of the samples synthesized by the hydrothermal route. (a) Nanoribbon at 160°C, (b) nanowire at 180°C and (c) nanorod at 200°C. Vieira et al. Nanoscale Research Letters 2012, 7:310 Page 2 of 5 http://www.nanoscalereslett.com/content/7/1/310 combination and testing of new materials that can sense pH easily. In addition, the commercial high-input im- pedance device (FET part) in SEGFET sensors can be reused, since only the extended gate membrane has to be built [4,6,9,18]. The 1D V2O5�nH2O nanostructures deposited on Au-coated substrates were immersed in buffer solu- tions with different pH (pH from 2 to 12), and the output voltage of the operational amplifier was recorded over time. Figure 4a shows the dynamic re- sponse of all 1D V2O5�nH2O nanostructures to pH variations. Despite the structural changes due to the conditions of hydrothermal synthesis, the V2O5�nH2O synthesized at 160°C (in nanoribbon form with mono- clinic phase) and at 180°C (in nanowire form with orthorhombic phase) yielded similar results. The pH sensitivity of the 1D V2O5�nH2O nanostructures was determined based on the output voltage at 3 min. Within the limits of experimental error, the sensitivity did not change in any of the V2O5�nH2O morpholo- gies, indicating that the pH sensitivity is independent of the phase or nanostructure, as indicated in the inset in Figure 4b. The mechanism of pH sensitivity is due to the ampho- teric properties of the majority of metal oxides and can be explained by the well-known site-binding model [21,22]. According to this model, the surface of V2O5�nH2O nanostructures contains three sites, i.e., negatively charged groups, neutral groups and positively charged groups. The total surface charge can be altered by the formation of metal complexes on the surface of V2O5�nH2O nanostructures according to the following equation [21,22]: ψ ¼ 2; 3kT q β βþ 1 pHpzc � pH � � where pHpzc is the pH value at the point of zero charge, q is the elementary charge, k is the Boltzmann constant, T is the absolute temperature, and β is a factor that reflects the chemical sensitivity of the gate material. Modifications in the pH of the electrolyte cause changes in the concentration of protons, allowing for control of the output signal of SEGFET devices. The site-binding model is consistent with the experimental results, indi- cating that the value of β is the same for any V2O5�nH2O morphologies. The pH sensitivity of 1D V2O5�nH2O nanostructures is consistent with the theoretical Nernstian value expected for pH-sensitive materials (59.2 mV.pH^−1) and in excellent agreement with values reported for other metal oxide pH-sensing membranes [6-10]. In addition, due to this property, 1D V2O5�nH2O nanostructures can be applied as field-effect based biosensors, since the biomolecule-catalyzed reaction changes the ion concen- tration in solution, as suggested in the literature [23]. Conclusions In summary, we have reported the results of an investiga- tion of vanadium pentoxide nanostructures as sensitive a b c Figure 3 FE-STEM images of a 1D V2O5.nH2O nanostructures synthesized. (a) 160°C, (b) 180°C and (c) 200°C. Vieira et al. Nanoscale Research Letters 2012, 7:310 Page 3 of 5 http://www.nanoscalereslett.com/content/7/1/310 material in SEGFET pH sensors. The use of the hydro- thermal route combined with FET-based sensors yielded nanometric pH-sensitive materials. 1D V2O5�nH2O nanostructures showed pH sensitivity close to the theor- etical value. Despite the influence of the synthesis temperature on the morphological and structural proper- ties of the material, its pH sensitivity remained un- affected, as expected. Our strategy shows potential advantages for the construction of low-cost pH sensing membranes with promising applications in field effect- based biosensors. Competing interests The authors declare that they have no competing interests. Authors’ contributions NCSV conceived the study, contributed with its design and coordination, and drafted the manuscript. WA, CR and VRM synthesized all vanadium pentoxide nanostructures, and they were responsible for its characterization. AF made the films and helped the experiments related to the pH sensor. FEGG gave advice and guided the experiments. All authors read and approved the final manuscript. Acknowledgments The authors acknowledge CAPES, CNPq and FAPESP for their financial support of this research. 0 3 6 9 12 15 18 21 -100 0 100 200 300 400 500 600 200 °C - nanorods Time / min pH 12 pH 10 pH 8 pH 7 pH 6 pH 4 V ol ta ge / m V pH 2 0 3 6 9 12 15 18 21 -100 0 100 200 300 400 500 600 180 °C - nanowires V ol ta ge / m V Time / min pH 2 pH 4 pH 6 pH 7 pH 8 pH 10 pH 12 0 3 6 9 12 15 18 21 -100 0 100 200 300 400 500 600 160 °C - nanoribbons V ol ta ge / m V Time / min pH 2 pH 4 pH 6 pH 7 pH 8 pH 10 pH 12 a 2 4 6 8 10 12 -100 0 100 200 300 400 500 600 160 180 200 52 56 60 V ol ta ge / m V pH value b S en si tiv ity / m V .p H -1 Temperature / oC Figure 4 Dynamic response of all 1D V2O5�nH2O nanostructures to pH variations. (a) Typical dynamic response of 1D V2O5�nH2O nanostructured sensing membranes to variations in pH and (b) pH sensitivity calculated at 3 min. Inset: pH sensitivity of 1D V2O5�nH2O nanostructures as a function of hydrothermal synthesis temperature. Vieira et al. Nanoscale Research Letters 2012, 7:310 Page 4 of 5 http://www.nanoscalereslett.com/content/7/1/310 Author details 1Departamento de Física e Ciências dos Materiais, Instituto de Física de São Carlos, Universidade de São Paulo, Avenida Trabalhador São-carlense 400, São Carlos, São Paulo CP 369/13560-970, Brazil. 2Departamento de Físico-Química, Instituto de Química de Araraquara, Universidade Estadual Paulista Júlio de Mesquita Filho, Rua Prof. Francisco Degni 55, Araraquara, São Paulo CP 355/14801-907, Brazil. 3Embrapa, Empresa Brasileira de Pesquisas Agropecuárias, Rua XV de Novembro 1452, São Carlos, São Paulo 13560-970, Brazil. Received: 18 January 2012 Accepted: 30 May 2012 Published: 18 June 2012 References 1. 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Schoning MJ, Poghossian A: Recent advances in biologically sensitive field-effect transistors (BioFETs). Analyst 2002, 127:1137–1151. doi:10.1186/1556-276X-7-310 Cite this article as: Vieira et al.: Ion-sensing properties of 1D vanadium pentoxide nanostructures. Nanoscale Research Letters 2012 7:310. Submit your manuscript to a journal and benefi t from: 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the fi eld 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com Vieira et al. Nanoscale Research Letters 2012, 7:310 Page 5 of 5 http://www.nanoscalereslett.com/content/7/1/310 Abstract Background Methods Results and discussion link_Fig1 link_Fig2 Conclusions link_Fig3 Competing interests Authors´ contributions Acknowledgments link_Fig4 Author details References link_CR1 link_CR2 link_CR3 link_CR4 link_CR5 link_CR6 link_CR7 link_CR8 link_CR9 link_CR10 link_CR11 link_CR12 link_CR13 link_CR14 link_CR15 link_CR16 link_CR17 link_CR18 link_CR19 link_CR20 link_CR21 link_CR22 link_CR23