Synthesis of highly structured spherical Ag@Pt core-shell NPs using bio-analytes for electrocatalytic Pb(II) sensing

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Highlights

  • Development of a bio-inspired route for the synthesis of highly structured and stable Ag@Pt core-shell NPs.

  • Ag@PtNPs catalyzed electrochemical Pb(II) sensing from tap water, river water and sewage water.

  • Sensitivity and limit of Pb(II) detection showed clear superiority over AgNPs and PtNPs catalyzed system.

  • Response of Pb(II) sensor was almost invariant in the presence of Cu(II), Cd(II) and Hg(II) co-ions.

Abstract

This research work is taken up to synthesize highly crystalline and ordered Ag@PtNPs core-shell nanoparticles (NPs) via a bio-inspired route using Psidium guajava leaves-extract with high ascorbic acid equivalent (284 /100 g fresh leaves). A short microwave exposure (900 W and 300 s) led to a quick formation of AgNPs-seed (25.7 nm avg). The seed was then subjected for a rapid growth (240 s) of Pt-shell (4.55 nm) which was synergized by bio-extract and microwave irradiation forming Ag@PtNPs. The graphite electrode was surface-modified using these NPs for electrocatalytic sensing of Pb(II) by square wave anodic stripping voltammetry. The electron transfer resistance of bare graphite was reduced by 1.2, 26, and 2.9 folds, respectively, with PtNPs, AgNPs, and Ag@PtNPs modification. The sensitivity of Ag@PtNPs/graphite electrode for Pb(II) sensing calculated against the electroactive area was about 4.5 and 20.2 folds higher than PtNPs/graphite and AgNPs/graphite electrodes at −0.43 V vs. Ag/AgCl and pH 5. The limit of detection was as low as 0.8 nM. The response of the sensor (max. 1.98 % variation in peak current) was almost invariant in the presence of co-ions (Cd(II), Cu(II) and Hg(II) from 0.25 to 10 μM). The sensor it could effectively determine Pb(II) from environmental water samples collected from river, domestic supply and sewage treatment plant.

Introduction

Heavy metals (HMs) contamination affects both aquatic and terrestrial environments [1]. The main sources of HMs in the environment include uncontrolled anthropogenic activities [[2], [3], [4]]. HMs such as cadmium (Cd), lead (Pb), mercury (Hg), chromium (Cr), arsenic (As), etc. are known to be bio-accumulated in living organisms including humans. The presense of a minute concentration of such HMs causes many metabolic disorders and disfunctioning of central nervous system, kidney, gastro-intestinal, etc [3,5].

HMs are considered as most dreadful contaminants present in water and food. The presence of such HMs, thus is strictly regulated by authorities, namely World Health Organization (WHO), United States Environmental Protection Agency (USEPA) and Bureau of Indian Standards (BIS). For example, the maximum permissible limit of Pb(II) in drinking water is set as 48, 72, and 48 nM, respectively, by the aforementioned organizations [6,7]. The adherence to the permissible limit necessitates the requirement of highly sensitive, selective, and reliable methods for the detection of HMs (at ppb level) in water, wastewater, soil, biological samples, food, etc.

The sophisticated techniques, such as Atomic Absorption Spectrometry (AAS), X-Ray Fluorescence (XRF) [8], Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) [9,10], etc., require heavy and expensive installations, trained human resources, and complex sample-preparation procedures [11]. Among these, ICP-MS could determine a metal ion at an extremely low concentration over AAS and XRF.

On the other hand, electrochemical methods are rather simple, inexpensive, and sensitive. Most importantly, they can be made portable and utilized in and on-field applications, and thus, can eliminate the inconvenience of storage and handling of samples [12]. However, the sensitivity of HMs detection in electrochemical techniques using bulk electrodes is usually lower (9.5–31.4 nA μg−1∙L−1) and the response is also sluggish [13]. The modification of the electrode composition using nanostructured materials could endow a rapid and accurate detection of HMs with high sensitivity [12].

Nanocatalysts such as carbon nanoparticles (NPs) [14], carbon nanotubes [15], nanowires [16], graphene-based materials [17], noble metal nanoparticles [18], etc. have been extensively used in modifying conventional electrodes like glassy carbon electrodes, graphite paste electrodes or metal electrodes to increase their specificity and sensitivity towards the detection of target metal ions. Moreover, NPs are known to eliminate the memory effect and help improve the selectivity of the electrode towards a specific HM when it is present in a mixture of interfering HMs [19]. Moreover, NPs-modified electrodes are known to show an improved limit of detection (LOD) [20]. Among the various methods of synthesis of metal NPs, bio-inspired methods minimize the use of environmentally aggressive chemicals [[21], [22], [23], [24]]. However, it suffers from being a slow process; the structure and the size of the particles are also not uniform [25]. These are the main challenges for the application of bio-inspired processes for the synthesis of tailor-made nanoparticles such as core@shell structures [26]. We thought that, biomolecules could easily cap (i.e. 7–10 % w/w of NPs) on the surface of the preformed core, and it hinders the formation of the shell on the top of the core metal.

Thus, we work, herein, a bio-inspired pathway using Psidium guajava (guava) leaves-extract [27,28] which is rich in various electron donor bio-analytes such as chlorogenic acid derivatives, d-glucose, quercetin, etc. with a high ascorbic acid equivalent (AA equivalent 284 per 100 g fresh leaves). Therefore, we have selected guava leaves-extract for the synthesis of highly ordered structure Ag@Pt core-shell NPs. Moreover, the bio-extract is merited with self-stabilization of NPs [29,30].

The rate of the reaction for the formation of NPs is accelerated by the microwave irradiation. A protocol is devised for the partial removal of the capping layer from the top of the Ag-core without significant particles aggregation. Subsequently, a Pt-shell is formed on the Ag-core using the same bio-analytes. These Ag@Pt core-shell NPs are surface embedded in the graphite support electrode for the electrocatalytic Pb(II) determination. AgNPs and PtNPs synthesized in a single step bio-inspired process were also tested to understand the catalytic activity of Ag@PtNPs for Pb(II) determination, and the mechanism of enhanced Pb(II) sensing catalysed by Ag@PtNPs over its pure metal NPs counterparts is also explored.

Square wave anode stripping voltammetry (SWASV) technique was employed for the detection of ultra-trace Pb(II) ions. In SWASV, Pb(II) was first electrodeposited in an acetate buffer (pH 5) at the optimized deposition potential and time [12]. The optimal condition for the stripping of Pb from the working electrode was then found out for the determination of Pb(II) in pure and mixed matrix water samples. Furthermore, the interference of co-existing Cd(II), Cu(II), and Hg(II) was studied in details.

Section snippets

Reagents

AgNO3 (CAS No: 7761-88-8), HNO3 (65 % emsure), NaOH (CAS No: 1310-73-2), K₃[Fe(CN)₆] (CAS No: 13746-66-2), trisodium citrate (Na3C6H5O7, CAS No: 6132-04-3), sodium citrate (CH3COONa, CAS No: 6131-90-4), acetone (C3H6O, CAS No: 67-64-1), isopropyl alcohol (C3H8O, CAS No: 71-23-8), and paraffin wax (melting point >58 °C) were procured from Merck, Mumbai, India. Acetic acid (CH3COOH, HPLC grade), Ascorbic acid (C6H8O6, CAS No: 50-81-7), 2,2-diphenyl-1-picrylhydrazyl (C18H12N5O6, DPPH, CAS No:

UV–vis absorption spectra of AgNPs, PtNPs and Ag@PtNPs

Ag-core, or simply AgNPs at the beginning just before the addition of Pt-precursor, showed a strong absorption peak at around 408 nm due to the well known surface plasmon resonance (SPR) effect of these NPs (Fig. 2a). At the end of the reaction, the spectral response became similar to that of the pure PtNPs solution ensuring proper surface coverage of AgNPs by PtNPs. In case of Ag@PtNPs, with the development of Pt-shell, a blue shift (381 nm) was observed (Fig. 2a), and the SPR peak intensity

Conclusions

We have successful devised a new bio-inspired method for the synthesis of seed-mediated Ag@PtNPs core-shell NPs using the bio-extract of Psidium guajava leaves and microwave irradiation. The size of, thus synthesized, AgNPs, PtNPs, and Ag@PtNPs (14.5 nm core diameter and 4.55 nm shell thickness) are 25.5, 19.5, and 24.5 nm. These NPs were successfully decorated on graphite support electrode for the electrocatalytic Pb(II) sensing. The Pb(II) deposition potential and deposition time on the

CRediT authorship contribution statement

Smruti Ranjan Dash: Methodology, Investigation, Data curation, Software, Writing - original draft. Subhendu Sekhar Bag: Supervision, Validation, Formal analysis. Animes Kumar Golder: Conceptualization, Supervision, Resources, Validation, Visualization, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Animes Kumar Golder is Professor in Chem Eng Deptt at Indian Institute of Technology Guwahati. Over the years, he has made substantial contributions in photocatalysis, nanotechnology, sensor development, and AOPs. Dr. Golder has supervised many doctoral and master students. He is the author of several journal papers and conference proceedings. Dr. Golder is the Principal investigator of various sponsored projects funded by DST, CSIR, and DST-UKIERI. Dr. Golder is a BOYSCAST Fellow awarded by

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  • Cited by (0)

    Animes Kumar Golder is Professor in Chem Eng Deptt at Indian Institute of Technology Guwahati. Over the years, he has made substantial contributions in photocatalysis, nanotechnology, sensor development, and AOPs. Dr. Golder has supervised many doctoral and master students. He is the author of several journal papers and conference proceedings. Dr. Golder is the Principal investigator of various sponsored projects funded by DST, CSIR, and DST-UKIERI. Dr. Golder is a BOYSCAST Fellow awarded by DST to visit Civil and Environ Eng Deptt at University of Delaware (USA).

    Subhendu Sekhar Bag, PhD, (Bioorganic Chemistry), joined the Department of Chemistry at Indian Institute of Technology Guwahati, INDIA as an Assistant Professor in the year of 2008. He is now working as a Full time Professor since 2017 and is continuing to explore Bio-organic Chemistry of Nucleic Acids, Peptides, and b-Lactam Antibiotics, especially small biological pharmacophores, Bio-inspired nanoparticle synthesis and applications.

    Smruti Ranjan Dash is currently pursuing his PhD at the Centre for the Environment, Indian Institute of Technology Guwahati. His main research interest concentrates on the bio-inspired synthesis of nanoparticles and their application in developing electrochemical sensors for the detection of various organic and inorganic water pollutants.

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