Title : Direct and selective quantification of Cr VI in waste waters with raffinose capped silver nanoparticles as sensitive optical sensor
In the present study, a new method for direct and selective determination of Cr(VI) in the presence of Cr(III) in water samples is presented using raffinose capped silver nanoparticles (Ag/Raff NPs). The nanoparticles used were obtained by one-step, one-phase “green” synthesis based on chemical reduction of Ag+ using D-(+) raffinose as both reducing and capping agent, and sodium hydroxide as a reaction catalyst. Careful kinetic studies and optimization of the response time of the optical sensor ensured selective, accurate and reliable determination of Cr(VI) in waters in only 5 min. The method is based on the variation of LSPR absorption band intensity as a result of electrostatic interaction between the negatively charged Ag/Raff NPs and positive Cr(III) ions, in-situ produced by chemical reduction of Cr(VI) with ascorbic acid at pH 4, combined with the fast kinetics of Cr(III) coordination to the –OH groups of capping agent on the nanoparticle surface, causing further the nanoparticle aggregation. A standard addition calibration method for Cr(VI) quantification in real water samples, based on systematic study of matrix interferences, is proposed.
Analytical figures of merit, the validation method and application to real samples are presented. The calibration curve for Cr(VI) is linear in the range 2.5 - 7.5 μmol L-1, limit of quantification achieved is 1.9 μmol L-1, and values of relative standard deviation vary from 3 to 5 % for concentration level 1.9 - 7.5 μmol L-1.
The interference studies performed in the presence of various metal ions show very good selectivity of Ag/Raff NPs toward Cr(VI) species. The observed selectivity of the optical response of raffinose capped silver nanoparticles towards Cr(VI) is achieved because of optimally selected combination of two key parameters in the procedure: in situ reduction of Cr(VI) with ascorbic acid in the presence of raffinose capped silver nanoparticles and optimal contact time of 5 min between the analyte and the optical probe.
The added-found method is used to confirm accuracy and precision of developed analytical approach. The analytical procedure was validated by comparative analysis of underground waters, polluted with Cr(VI) from chromium plating factory, with the standard method based on spectrophotometry using diphenylcarbazide (ISO 11083: Water Quality —Determination of Chromium(VI)—Spectrometric Method Using 1,5-Diphenyl- carbazide). Very good agreement between the results obtained undoubtedly validates the analytical probe developed (raffinose capped silver nanoparticles) for selective quantification of Cr(VI) and confirms the applicability of the developed analytical procedure for routine application in the laboratory practice. The analytical approach developed might be used also as a fast screening method on site.