SnO2 Quantum Dots-Reduced Graphene Oxide Composite for Enzyme-Free Ultrasensitive Electrochemical Detection of Urea Dipa Dutta,† Sudeshna Chandra,† Akshaya K. Swain,‡ and Dhirendra Bahadur*,† †

Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, Mumbai, Maharashtra 400076, India ‡ IITB Monash Research Academy, Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, Mumbai, Maharashtra 400076, India S Supporting Information *

ABSTRACT: Most of the urea sensors are biosensors and utilize urease, which limit their use in harsh environments. Recently, because of their exceptional ability to endorse faster electron transfer, carbonaceous material composites and quantum dots are being used for fabrication of a sensitive transducer surface for urea biosensors. We demonstrate an enzyme free ultrasensitive urea sensor fabricated using a SnO2 quantum dots (QDs)/reduced graphene oxide (RGO) composite. Due to the synergistic effect of the constituents, the SnO2 QDs/RGO (SRGO) composite proved to be an excellent probe for electrochemical sensing. The morphology and structure of the composite was characterized by various techniques, and it was observed that SnO2 QDs are decorated on RGO layers. Electrochemical studies were performed to evaluate the characteristics of the sensor toward detection of urea. Amperometry studies show that the SRGO/GCE electrode is sensitive to urea in the concentration range of 1.6 × 10−14−3.9 × 10−12 M, with a detection limit of as low as 11.7 fM. However, this is an indirect measurement for urea wherein the analytical signal is recorded as a decrease in the amperommetric and/or voltammetric current from the solution redox species ferrocyanide. The porous structure of the SRGO matrix offers a very low transport barrier and thus promotes rapid diffusion of the ionic species from the solution to the electrode, leading to a rapid response time (∼5 s) and ultrahigh sensitivity (1.38 μA/fM). Good analytical performance in the presence of interfering agents, low cost, and easy synthesis methodology suggest that SRGO can be quite promising as an electroactive material for effective urea sensing.


nanocrystals on the graphene surface is quite challenging due to the uneven distribution of functional groups on its surface.4,5 Presently, tin oxides/graphene composites have aroused the interest of researchers, since they are efficient composites for lithium ion batteries,4,6 supercapacitors,7 dye degradation,8 and biosensing.9 The SnO2/graphene composites can be used to construct electrochemical sensors due to high active surface area, high electrocatalytic activity, chemical stability, and interface dominated properties. In line with this, one may think of using the composites for sensing small molecules like urea which is an end product of protein degradation and nitrogen metabolism. Urea is also a critical indicator of liver and kidney malfunction in the human body.10 The normal concentration of urea in blood serum is 3−7 mM (15−40 mg/dL).11 The presence of urea in the environment is also a matter of great concern for ecosystem conservation and human health protection.12 Due to its relatively higher water solubility,

omposite materials containing two or more constituents are of great interest as they can jointly exhibit unique physical and chemical properties for varied applications. Quantum dots are zero dimensional materials having sizedependent properties which make them quite interesting. Among the wide band gap semiconductors, tin dioxide quantum dots (SnO2 QDs) have attracted enormous research interest for their widespread applications in electronics and optics due to their excellent electrical and electrochemical properties.1,2 On the other hand, graphene has also attracted great attention due to its outstanding electrical, thermal, and optical properties and a theoretical high surface area.3,4 The number of graphene layers in reduced graphene oxide (RGO) significantly influences the properties of RGO. However, these graphene layers tend to aggregate in solution due to lack of oxygen-containing groups which can be overcome by incorporating different metal oxide nanoparticles between the layers.4 Metal oxide/graphene composites are expected to exhibit excellent material properties of parent components, due to the synergistic effect of both. Uniform loading of metal oxide © XXXX American Chemical Society

Received: February 25, 2014 Accepted: May 15, 2014

A | Anal. Chem. XXXX, XXX, XXX−XXX

Analytical Chemistry


can be explained with the fact that the surface of the GO is covered with oxygen-containing groups, such as hydroxyl and epoxy at basal plane and carbonyl and carboxyl acid at the edges.22 Along with the reduction of GO, the hybrid complex ((N2H4)m(SnCl4)n) simultaneously decomposes to Sn4+,23 which gets attached to the surface of RGO through the oxygen-containing groups. The attached Sn4+ then get converted to SnO2 in an aqueous medium.24 The bare RGO was synthesized by the same method described above without addition of the hybrid complex [(N2H4)m(SnCl4)n]. The methodology for the preparation of bare SQDs has been described elsewhere.21 Materials Characterization. The crystallinity and phase purity of the products were verified by X-ray diffraction (XRD) patterns using a Philips powder diffractometer PW3040/60 with Cu Kα (1.5406 Å) radiation. The surface compositions and chemical states of the samples were determined by X-ray photoelectron spectroscopy (XPS) (MULTILAB from Thermo VG Scientific) using monochromatic Al Kα X-rays (1486.6 eV). The size and morphology of the samples were investigated using a high resolution transmission electron microscopy (HRTEM), JEOL JEM 2100F (200 kV), and field emission scanning electron microscope (FESEM, JEOL, JSM-7600F). Raman scattering measurement was performed at room temperature on Horiba Jobin Yvon LabRam series HR(800) spectrometer using an excitation of 514.5 nm from an Ar laser. The porosity and surface area measurement were performed after thoroughly degassing the samples at 150 °C for 4 h, using an N2 absorption desorption isotherm in a surface area and porosity analyzer (Micromeritics ASAP 2020). Electrochemical Studies. Electrochemical behavior of SRGO and its constituents (RGO and SQDs) were studied in detail. Cyclic voltammetry (CV), amperometry, and electrochemical impedance spectroscopy (EIS) analysis were conducted on a CH Instruments Model 660D electrochemical analyzer (CH Instruments Inc., US) using a three-electrode system with modified glassy carbon electrode (GCE) as the working electrode (3.14 mm2), a platinum wire as the counter electrode, and Ag/AgCl as the reference electrode in 0.1 M potassium ferricyanide/potassium ferrocyanide ([Fe(CN)6]3−/4−) couple solution. The EIS was performed in 0.1 M [Fe(CN)6]3−/4− solution at a potential of 0.32 V in a frequency range of 10−106 Hz at a signal amplitude of 10 mV. GCEs were polished with slurries of 1, 0.3, and 0.05 μm alumina powders and rinsed thoroughly with doubly distilled water. Then, the electrodes were sonicated alternatively in methanol and distilled water for 15 min, and the whole process was repeated six to seven times. A thin film of the materials (40 μg) was deposited on the surface of the working electrodes by the drop-casting method. The electrodes were left overnight to dry at room temperature. The electrodes were polished and cleaned before every deposition step.

a considerable amount of the applied urea herbicides is washed out in the aquatic environment. It can pollute the surface and the groundwater into which it drains.13 It is therefore, essential to analyze urea in the environment, drinking water, and food.12 Electrochemical analysis has inherent advantages of simplicity, high sensitivity, and relatively low cost over other sophisticated methods like high performance liquid chromatography, chemiluminescence, fluorimetry, and the like. Most of the urea sensors are biosensors, which utilize urease (Urs) as a sensing element. However, their complicated immobilization procedures, activity, stability, high cost of enzymes, and critical operating condition have some limitations, and they are not suitable in harsh environments.14 Recently, metal oxide matrices such as NiO nanoparticles,10 zinc oxide nanostructures,15 metal oxide−chitosan composite,16 carbonaceous materials composites,17 and quantum dots18 have been used for fabrication of a sensitive transducer surface for urea biosensors because of their exceptional ability to endorse faster electron transfer between the electrolyte and the electrode. So far, no attempt has been made to fabricate enzymeless chemical sensors for lower level urea detection. SnO2 QDs (SQDs) and RGO composite based chemical sensors can be used as potential probes for detection of urea in environmental samples. Herein, we report a new strategy for fabrication of SQDs/ RGO (SRGO) from a hybrid complex [(N2H4)m(SnCl4)n]. Different morphological characterization techniques were used to confirm the formation of the SRGO composite. Electrochemical studies were performed to investigate its performance as a proposed enzymeless chemical sensor for urea. It showed a detection limit which is much lower than the previously reported sensors that are based on different matrixes such as metal oxides, carbon materials, and quantum dots.16−18 SRGO has synergistic effects of the QDs and the two matrixes, namely, SnO2 and RGO, which results in highly sensitive sensor material.

EXPERIMENTAL SECTION Materials Preparation. A hybrid SRGO composite was fabricated in two simple steps. In the first step, graphite oxide (GO) was prepared from natural graphite power, and in the second step, it was reduced to form reduced graphene oxide. Natural graphite powder (trace metals 99.99%, average particle size

SnO(2) quantum dots-reduced graphene oxide composite for enzyme-free ultrasensitive electrochemical detection of urea.

Most of the urea sensors are biosensors and utilize urease, which limit their use in harsh environments. Recently, because of their exceptional abilit...
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