Monitoring of ppm level humic acid in surface water using ZnO–chitosan nano-composite as fluorescence probe
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Surface water contains natural pollutants humic acid (HA) and fulvic acid at ppm level which form carcinogenic chloro-compounds during chlorination in water treatment plants. We report here synthesis of ZnO–chitosan (CS) nano-composites by simple hydrothermal technique and examined their application potential as fluorescent probe for monitoring ppm level HA. These ZnO–CS composites have been characterized by HRTEM, EDX, FTIR, AFM and Fluorescence Spectra. HRTEM images show the formation of ZnO–CS nano-composites of average diameter of 50–250 nm. Aqueous dispersions of these nano-composites show fluorescence emission at 395 nm when excited at 300 nm which is strongly quenched by ppm level HA indicating their possible use in monitoring ppm level HA present in surface water.
KeywordsZnO nano-rods Monitoring HA in surface water Fluorescent probe
Surface water contains natural organic matter (NOM) in the form of humic acid (HA) and fulvic acid (FA) in the concentration range of 0.1–20 ppm (Rodrigues et al. 2009). Determination of humic substances (HS) such as HA and FA is important in water treatment plant as they form carcinogenic tri-chloro methane-type bi-products (Breider and Albers 2015) during chlorination of water. But estimation of HA using common analytical technique is difficult (Breider and Albers 2015) because of highly heterogeneous structure of HA (Piccolo 2002). Among the spectroscopic techniques, the UV method exploits the UV absorption property of HA at 254 nm (Khan et al. 2014) and IR technique determines the total organic carbon (TOC) using the principle of determination of CO2 concentration formed after complete combustion of the sample by infrared absorption spectroscopy. But both these techniques suffer from disadvantages such as agglomeration of HA which limits the applicability of Beer’s law in UV method and various pre-treatment requirements in TOC method. HA is a major component of HS formed by microbial decomposition of plant bio-mass. Chemically, it has a complex molecular structure (Piccolo 2002) containing both aromatic and aliphatic skeletons with ionisable –COOH and phenolic –OH groups. Because of the presence of these groups, the surface charge of HA colloid is negative (Angelico et al. 2014) (zeta potential is negative at neutral pH). Presence of HA in surface water and water from upland peat catchments in tropical climates is obvious, hence their monitoring and removal are essential prior to their treatment in drinking water plants.
In this communication, we report the preparation of Zn–CS nano-composites and explore the possibility of using these nano-composites as fluorescent probe for estimation of ppm level HA in surface water intended for drinking purpose. Previously, ZnO nanomaterials (NMs) have been used as (Khayatian et al. 2014) gas sensors. The surface charge of ZnO NMs may be tuned by coating with a positively charged polyelectrolyte as a capping agent.
We report a simple hydrothermal method of synthesis of ZnO nano-composites using depolymerised CS as coating material. CS has been selected as coating material because of two reasons (1) CS is a bio-safe and biodegradable polymer (Saber et al. 2010; Balamurugan 2012) easily obtained from natural polymer chitin; (2) this polyelectrolyte is positively charged (Schatz et al. 2004) at neutral pH, so that CS-capped ZnO will be an ideal material for interacting with negatively charged bio-colloid HA.
During the recent years (Park et al. 2002, Duan et al. 2001 and Huang et al. 2001), much attention has been paid to the synthesis of ZnO NMs such as nano wires, nano belts and nano-rods because of their diverse applications in electronic and photonic devices. Tuning of crystal growth in one direction by chemical synthesis is difficult (Wang et al. 2004) unless some anisotropic force acts in the growth directions. It is reported (Song et al. 2008) that the use of NH4Cl and Zn salt in alkaline (pH 11.0) hydrothermal synthesis leads to the formation of ZnO nano-rods; we have made an attempt to perform similar synthesis of ZnO nanorod–CS composite using acid-depolymerised CS which contains primary amine groups. It is anticipated that the ZnO–CS composites should have positively charged outer surface and will interact with negatively charged HA by electrostatic attraction. It is also anticipated that fluorescence of ZnO–CS composites (Sönmez and Meral 2012) will be quenched upon binding with HA.
Materials and method
Low-molecular weight CS was purchased from Sigma–Aldrich (Saint Louis, Missouri, USA) and used without further treatment. Zinc chloride (technical grade) was purchased from CQ concepts INC (Ringwood, Illinois, USA). Na salt of HA was purchased from Sigma–Aldrich and used as received. Ethanol (95 %) was received from Pharmco-AAPER (Brookfield, CT). ZnO nanomaterials (NMs) were purchased from BASF(Z-COTE® HP1) for using as control.
De-ionized (DI) water was obtained from Barnstead Nanopure Diamond purifier (Model number D11931).
Preparation of depolymerised CS-capped ZnO NPs
300 mg of CS and 300 mg of Zn(NO3)2 were taken in a hydrothermal tube containing 30 ml 1 % HCl solution and heated to 150 °C for 90 min; at this stage CS underwent depolymerisation and formed complexes with Zn2+. The solution was cooled to room temperature and its pH was adjusted to 11.0 by adding drop by drop 1.0 N NaOH solution in the presence of 100 mg NH4Cl (Song et al. 2008). The resulting solution was continuously heated at 150 °C under hydrothermal condition as before for 90 min. The solution was cooled and centrifuged at 5000 rpm for 10 min, the precipitate was washed with DI water and the process was repeated 5–6 times to remove unbound chitosan. The final precipitate was vacuum dried for 72 h to obtain powdered product.
HRTEM sample preparation
About 1 mg of ZnO powder sample was dispersed in 1 ml of 95 % (v/v) ethanol using a bath sonicator (Barnstead Lab-Line, model # 9322; 600 Watts). Then a drop of NM suspension was directly placed onto the carbon-coated side of a copper TEM grid. The TEM grid was kept on a piece of Whatman filter paper to absorb excess liquid from the drop. The grid was then air dried for 15 min followed by drying using a vacuum pump overnight.
UV–visible, fluorescence, FTIR and HRTEM studies
UV–vis absorption study was conducted to characterize absorption characteristics of the material to understand interaction of HA with coated ZnO NMs. UV–vis spectra of ZnO–CS composites were recorded using a Varian Cary 300 Bio UV–vis spectrophotometer. Fluorescence excitation and emission measurements of these samples were done using an SPEX Nanolog Spectrofluorimeter (Horiba Jobin–Yvon). Fourier Transform Infra-red Spectroscopy (FT-IR) technique was used to characterize the ZnO NMs and HA-bound NMs. FT-IR spectra were recorded on Perkin Elmer Spectrum 100 attenuated total reflection (ATR) FT-IR Spectrometer. HRTEM was taken with FEI Technai F30 TEM operated at 200 kV.
For AFM images, we used tapping mode operation of the equipment (Veeco Manifold multimode V model) using silicon nitride tip (radius B 50 nm) attached to a cantilever (spring constant = 0.032 Nm, oscillating frequency 0–600 kHz), and the sample was spin coated onto glass surface. All measurements were done at room temperature.
Results and discussions
It is interesting to note that fluorescence intensity of ZnO–CS is gradually quenched by HA till 24 ppm of HA concentration; the observed fluorescence quenching primarily indicates interactions of ZnO–CS with HA.But above this concentration of HA, no further quenching is noticed. This may be owing to completion of monolayer coverage of HA (Röcker et al. 2009) onto ZnO–CS surface.
In conclusion, we have synthesized water-dispersible ZnO–CS nano-composites by simple hydrothermal method and demonstrated that these NMs may be used as fluorescence sensors for the estimation of ppm level HA with estimation limit up to 24 ppm. This preliminary result indicates possible applicability of this technique in the estimation of HA in water treatment plants.
The authors are thankful to the Reviewers for their valuable suggestions. SBM is thankful to Professor S Seal, Director of NSTC, UCF for support and encouragement.
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