Microbial Preparation Of Nitrogen And Sulfur-doped Graphene And Study On Its Application In The Simultaneous Determination Of Ultratrace Heavy Metals

Graphene is a single layer of carbon atoms with a hexagonal arrangement in a two-dimensional honeycomb lattice. Graphene exhibits novel unique electrical, mechanical, thermal and optical properties due to its unique two-dimensional structure, and shows significant potential applications for nanoscale devices, lithium ion battery, solar cell, photocatalytic, electrochemical sensors or biosensor. Elements doping graphene is a practical way to effectively modify its structures, electrical properties and facilitate their applications. Although many investigations involving graphene materials doped by N have been reported and the S doping of G had been realized in the recent study, to our knowledge, graphene materials doped with both N and S have rarely been investigated. In this work, we report a green one-step approach to synthesize N and S-doped G (N, S-G) and further promising application as electrochemical sensors to detect ultratrace lead and cadmium.The graphene oxide (GO) was synthesized from graphite powder using a little modified method of Hummers, which was then reduced by microbial respiration of SRB under mild conditions. The obtained doped graphene is characterized with the UV-visible absorption spectroscopy(UV-vis), Fourier Transform InfraRed (FT-IR) spectra, thermo-gravimetric analysis(TGA), X-ray diffractometer (XRD), Raman spectroscopy (Raman), X-ray photoelectron spectroscopy(XPS), transmission electron microscopy(TEM), scanning electron microscopy(SEM), and so on. The reduction mechanism of graphene was studied. The N,S-G/Nafion composite electrode was prepared with N,S-G, Nafion and glass carbon electrode as a substrate electrode. So the method of Simultaneous determination of lead and cadmium in potable water based on differential pulse anodic stripping voltammetry (DPASV) with N,S-G/Nafion/GCE situ-plated in bismuth(Bi) was built. A series of experiments have been carried out to obtain optimized parameters, and experimental results were analyzed and discussed. The results are as follows:The N, S-G was characterized as the single-layer graphene by those characterization techniques, which is indicative of completion of most reduction of GO within that period. The synthesized N,S-G contained high concentration of nitrogen (about6.11at%), with the presence of pyridinic N, pyrrolic N and graphitic N incorporated into graphitic network. The calculated doping concentration of sulfur was about1.1%(at%), and the sulfur into graphene were doped in the form of-C-S-C-(78%) and another state of-C-SOX-C(22%).Compared with the bare GCE, Nafion/GCE, BiFE and N,S-G/Nafion BiFE under the same conditions, the sharpest and highest peak current for the target metal ions were obtained at the Nafion-(N,S-G) BiFE electrode due to the structure with enhanced interfacial electron-transfer properties. In order to investigate the effect of N and S doping on the electrochemical property of G, we also synthesized chemically reduced GO with (N-G) and without N-doping (CR-GO) as well as S-doping(S-G), in which the doped ones have the similar doping concentrations of N and S as N,S-G. The DPASV analytical characteristics of different electrodes showed that heteroatoms(N or S) doping into G leaded to more intensive peak current of Pb2+and Cd2+at N-G/Nafion (or S-G/Nafion) than that at CR-GO/Nafion. Compared with N-G/Nafion BiFE and S-G/Nafion BiFE, the electrochemical signals on N, S-G/Nafion BiFE were improved by about36%and23%for Pb2+, and110%and91%for Cd2+, respectively. The signal enhancement may be attributed to N atom and S atom dually doping into the carbon lattice of graphene, which may lead to a large number of active C atoms for accumulating more heavy metal ions. A series of optimum parameters are Deposition potential-1.2V, Deposition time300s, Bi3+concentration175μg/L, Increment0.008V, Pulse amplitude0.05V, Pulse width0.05s, Pulse period0.2s, Quiet time10s. Under the optimized condition, the resulting calibration plots are linear over the range from9.0to30.0μg/L for the simultaneous determination of Pb2+and Cd2+on the N,S-G/Nafion BiFE. The calibration curves and correlation coefficients are y(μA)=0.48x(μg/L)-2.22, r=0.999and y(μA)=1.01×(μg/L)+1.68, r=0.999for Pb2+and Cd2+, respectively. The detection limits of N, S-G/Nafion BiFE were0.016μg/L for Cd2+and0.018μg/L for Pb2+, respectively. This method is simple and accurate, and worth to be further studied.