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The article describes a method for preconcentration and speciation of Cr(III) and Cr(VI) in river waters. It is based on ultrasound-assisted magnetic solid phase extraction employing a nanocomposite prepared from magnetite nanoparticles, manganese oxide and alumina oxide, and then funct i o n a l i z e d w i t h [ 3 -( 2 -am i n oe t h y l a m i n o ) p r o p y l ] trimethoxysilane (AAPTMS). By taking advantage of the oxidative properties of MnO 2 , the Fe 3 O 4 @MnO 2 ,Al 2 O 3 nanocomposite was used for the preconcentration of total chromium. The AAPTMS-modified nanocomposite, in turn, is selective for Cr(VI). The concentration of Cr(III) can be calculated as the difference between total chromium and Cr(VI). The preconcentrated chromium species were quantified using ICP-OES. The method was optimized using response surface methodology. Under optimal conditions, the limit of detection and quantification are 20 and 50 pg ⋅ mL −1 , respectively. The intraday and interday precisions of the method, expressed in terms of relative standard deviation, are 2.6 and 4.9%, respectively. In addition, the effects of potentially interfering ions were examined. The method was successfully applied to the speciation of chromium in spiked river water and a certified reference material.

The elevated concentrations of various trace metals beyond existing guideline recommendations in water bodies have promoted research on the development of various electrochemical nanosensors for the trace metals’ early detection. Inspired by the exciting physical and chemical properties of nanomaterials, advanced functional nanocomposites with improved sensitivity, sensitivity and stability, amongst other performance parameters, have been synthesized, characterized, and applied on the detection of various trace metals in water matrices. Nanocomposites have been perceived as a solution to address a critical challenge of distinct nanomaterials that are limited by agglomerations, structure stacking leading to aggregations, low conductivity, and limited porous structure for electrolyte access, amongst others. In the past few years, much effort has been dedicated to the development of various nanocomposites such as; electrochemical nanosensors for the detection of trace metals in water matrices. Herein, the recent progress on the development of nanocomposites classified according to their structure as carbon nanocomposites, metallic nanocomposites, and metal oxide/hydroxide nanocomposites is summarized, alongside their application as electrochemical nanosensors for trace metals detection in water matrices. Some perspectives on the development of smart electrochemical nanosensors are also introduced.

Highlights The influence of parameters on ultrasonic exfoliation of the NiFe LDH in pure water was studied. Various Concentration, amplitude, sonication times were investigated for exfoliation. The optimum condition of ultrasonic exfoliation of 60%, 10 mg mL −1 and 15 min. The best OER activity shows 250 mV overpotential at a current density of 10 mA cm −1. Enriched Fe 3+ were found on the surface of the exfoliated NiFe LDH.