Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Highly selective rapid colorimetric sensing of Pb2+ ion in water samples and paint based on metal induced aggregation of N-decanoyltromethamine capped gold nanoparticles
Graphical abstract
Introduction
Lead (Pb) has been used for centuries in different fields, including construction, plumbing, military, painting, sculptures etc. [1]. In 21st century, Pb has been widely used in lead-acid batteries [2]. Though beneficial, Pb has been recognized as one of the toxic elements. It is noted that the exceeding levels of Pb > 10 μg/dL in human blood affect the central and peripheral nervous system, resulting in behavioural problems, learning disabilities, hearing and speaking deficiencies [3,4]. European Food Safety Authority (EFSA) issued warning that even a low concentration of Pb (< 2.1 μg/L) causes adverse effect in children's intelligence development [5]. Higher levels of Pb > 70 μg/dL causes catastrophic health problems, including seizures, coma and finally death [6,7]. In modern days, Pb contamination mainly arises from gasoline, paints, toys, coal burning, auto emissions, and batteries [8]. The lead leaches out from these sources may enter into the water bodies and becomes a resource for human exposure [9,10]. The detection of Pb in the environmental bodies is difficult, because Pb is colourless and odourless. The currently available detection method for Pb are flame atomic absorption spectrometry (FAAS), electro thermal atomic absorption spectrometry (ET-AAS), inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma optical emission spectrometry (ICP-OES) [[11], [12], [13]]. Among these methods, the Environmental Protection Agency (EPA) approved only ICP-MS for testing Pb in national laboratories. The method is highly sensitive, but very expensive, required complicated sample preparation, trained technician and sophisticated instrument [14]. Thus, there is huge demand for a simple, rapid, and cost-effective system for Pb2+ detection.
Of late, nanoparticles (NPs) derived from coinage metals like silver, gold, and copper has been widely tested for developing colorimetric sensor for heavy metals [15]. The surface plasmon resonance (SPR) exhibit by these coinage metal NPs was used as a ruler in sensing experiments [[16], [17], [18]]. In our continuous effort to develop a colorimetric sensor for hazardous ions, we reported sulfide sensor based ciprofloxacin capped silver NPs (AgNPs), Hg2+ sensor based on N-acylethanolamine capped AgNPs, Au3+ sensor based on N-acyltromethamine capped AgNPs [[19], [20], [21]]. It was noted from our studies and other reports that the sensing property of the AgNPs differs significantly with respect to the capping agent [[19], [20], [21], [22], [23]]. Recently, the SPR properties of the gold nanoparticles (AuNPs) have been explored in colorimetric sensing based on aggregation or deaggregation by the target molecules [[24], [25], [26]]. Anwar et al., reported that the pyrazinium thioacetate capped AuNPs detects Pd2+ with limit of detection (DL) of 4.23 μM [27]. Su et al. reported selective Hg2+ sensor using thioctic acid-AuNPs with DL of 10 nM [28]. Dong et al., demonstrated chromium sensing based on aggregation-induced colour change in gallic acid-AuNPs [29]. Ratnarathorn et al. developed a maleic acid functionalized AuNPs to detect Pb2+ in 15 min with DL of 0.5 μg/L [30]. These literatures suggest that the surface functionalization of NPs impart different selectivity and sensitivity in colorimetric sensing. Driven by the demand, NPs based Pb2+ sensor system are reported, but many of them stopped at laboratory testing and a few have been shown to sense Pb2+ in real samples [31]. Considering the toxicity of lead and its widespread existence in houses in the form of batteries and paints urges the research community to develop rapid, selective sensor for lead. With that aim, we focussed on AuNPs and synthesized them using N-decanoyltromethamine (NDTM) as capping agent. NDTM falls under the category of N-acyl fatty acid amides. These classes of amides are known in lipodomics as lipid signalling molecules. NDTM is amphiphilic in nature, which undergoes self assembly in aqueous solution to form interdigitized vesicles [32]. The AuNPs formed in NDTM vesicle is highly stable, but undergoes rapid colour change from pink to violet in the presence of Pb2+ ions. The colour change was the result of a change in the aggregation state of NDTM-AuNPs, which was monitored by absorption spectroscopy, and confirmed by particle size analyser. Interference studies revealed that the NDTM-AuNPs are highly sensitive to Pb2+ ions. The proposed sensor works rapidly with DL of 0.35 μM. Based on the change in SPR maximum of NDTM-AuNPs, a ratiometric sensor was reported for sensing Pb2+ ions in sewage water, tap water, and also in commercial paints.
Section snippets
Materials
Decanoyl chloride, gold chloride was purchased from Sigma-Aldrich. Tromethamine, sodium hydroxide and were purchased from Merck. Sodium borohydride was obtained from Himedia. The chemicals and solvents used in this study are of analytical grade and used as received from the local supplier. Double distilled water was used throughout the study.
Synthesis of gold nanoparticles
The N-decanoyltromethamine (NDTM) as a capping agent was synthesized via the condensation reaction of tromethamine and decanoyl chloride, according to
Synthesis of gold nanoparticles
N-decanoyl tromethamine (NDTM) suspended in water undergoes self-assembly with critical aggregation concentration of 1.13 mM [32]. The self-assembled NDTM serves as an ideal template to stabilize gold NPs. The addition of sodium borohydride to an aqueous solution of gold chloride and NDTM produces a pink colour solution, indicates the formation of NDTM-AuNPs (inset of Fig. 1). The absorption spectra of NDTM-AuNPs show the characteristic surface plasmon resonance (SPR) maximum at 525 nm (Fig. 1
Conclusions
In summary, highly sensitive colorimetric sensor for Pb2+ has been developed using NDTM stabilized NDTM-AuNPs. The sensing depends on the colorimetric change from pink to violet colour, which is quite simple, rapid, cost-effective and does not require sophisticated instrumentation. The mechanism of sensing was based on aggregation induced change in the SPR band intensity of NDTM-AuNPs. The naked detection limit is 10 μM and spectrophotometric DL is 0.35 μM. Interference studies proved that the
CRediT authorship contribution statement
Sengan Megarajan: Investigation, Formal analysis, Writing - original draft. Kamlekar Ravi Kanth: Investigation. Veerappan Anbazhagan: Conceptualization, Methodology, Formal analysis, Writing - original draft.
Declaration of competing interest
There are no conflicts to declare.
Acknowledgements
SM gratefully acknowledges Teaching Assistantship from SASTRA Deemed University. The central research facility (R&M/0021/SCBT-007/2012-13), SASTRA Deemed University is acknowledged for the infrastructure. We thank VIT, Vellore for HRTEM facility.
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