An elegant and handy selective sensor for ppt level determination of mercury ions

Abstract

We project a reduced graphene oxide-fluorescein sensor moiety that offers a quick, selective and sensitive optical response to the presence of a noxious heavy metal ion, mercury (Hg²⁺) in aqueous solution. A turn off and turn on optical response is envisaged by the unit on competitive adsorption of fluorescein and mercury ions on reduced graphene oxide sheets. The crux behind the sensing of Hg²⁺ ion by the reduced graphene oxide-fluorescein unit lies behind the decrease in absorbance/fluorescence intensity of the dye on complexing with reduced graphene oxide and the corresponding restoration of the optical response upon highly selective competitive adsorption of Hg²⁺ ions by reduced graphene oxide sheets. Quantification of these optical responses was attained by colorimetric and fluorimetric measurements, which yielded ppb level and ppt level mercury ion assay. The LOD values were 240 ppb and 780 ppt respectively for these analytical methods. The moiety is highly selective and specific for mercury ions over alkali, alkaline earth metals and its congeners. The significant advance in this investigation is the fabrication of a reduced graphene oxide-fluorescein unit as a qualitative read out tool for naked eye detection of Hg²⁺ ions in ppm levels noting the colour and for the visual detection of ppt levels of Hg²⁺ through fluorescence in UV light. A highly efficient GFU immobilized filter paper strip was developed which could sense Hg²⁺ under UV light in a concentration as low as ppb level. Moreover, investigations on tap water samples spiked with Hg²⁺ ions underlines that the reduced graphene oxide-fluorescein moiety can swiftly monitor Hg²⁺ levels in complex solutions and demonstrates its potential utility in practical applications.

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Introduction

Real time monitoring of heavy metals in environmental, biological and industrial samples has gained significant attention over the past few decades and continues to be a hot area of research. All those metals whose density is higher than 3.5 g cm⁻³ fall under the category of heavy metals and are noted for their potential toxicity even at low thresholds. Among heavy metals, mercury, lead and cadmium are the topmost miscreants. Hg²⁺, ranked sixth, is a cumulative toxicant of global concern owing to its non biodegradability, long range transport in the atmosphere, persistence in the environment and its biomagnification through the food chain.^1–4 Hg²⁺ is released into the environment via both natural and anthropogenic sources. Hg²⁺ has far reaching effects on human health and can lead to dysfunctioning of the central nervous system, renal system failure, developmental delay in foetus, blindness, deafness etc. Mounting evidence from various studies underlines the adverse influence of Hg²⁺ ion on human health, which demands the trace detection of Hg²⁺ ions.^5–7 Though the detrimental effect of mercury ion has roused serious concern, the world's contamination with this toxic metal has increased in an alarming rate over the past few decades. The actual peril associated with Hg²⁺ arises when once they are introduced into aquatic bodies, as mercury ion will be solvated instantaneously to form solvated Hg²⁺ which is one of the most stable form of inorganic mercury. Hence there is always an urge to develop a selective, sensitive and efficient Hg²⁺ sensors in aqueous systems.

Towards this goal, apart from, selective cold vapor atomic fluorescence spectrometry,⁸ inductively coupled plasma mass spectrometry (ICP-MS)⁹ and atomic absorption/emission spectroscopy,¹⁰ quite a good quantum of sensors based on organic chromophores,¹¹oligonucleotides,¹²conjugated polymers,¹³ quantum dots,¹⁴ membranes,¹⁵ proteins,¹⁶ DNAzymes,¹⁷ antibodies,¹⁸ and nanomaterials¹⁹ have been fabricated for electrochemical and optical detection of Hg²⁺ ion in aqueous solution. ppb levels of Hg²⁺ ion detection can be achieved using instrumental intensive techniques. But their excellent performance is attained at the expense of time, labour and cost. The well known chemodosimeters for mercury ion rely on the changes in fluorimetric and colorimetric response in presence of the analyte ion. Several challenges still exists in this type of chemodosimeters also, namely high cost, complex processes, need for labeling agents, ease of hydroxylation, irreversibility, low selectivity and sensitivity, desulphurization etc. For e.g. Hg²⁺ based electrochemical sensors on gold operate at high temperatures and demand complicated electronic circuit,^20,21 likewise optical organic sensors can function only in organic solvents limiting their practical utility. Majority of the sensors often need a long equilibration time for detection; use of bio molecules for monitoring Hg²⁺ levels require the use of buffering solution, which also suffers from the interference from common ions and reduced efficiency.²² Hence any attempt in this direction that reduces all the aforesaid backlogs is significant and always laudable.

Currently, the star among carbon allotropes, graphene (G) has set an unbeatable benchmark in all spheres of technological and scientific research on account of its captivating properties.²³ Graphene based sheets like graphene oxide (GO) and Reduced Graphene Oxide (RGO) have envisaged its role as elite systems for quantitative detection of many bio molecules,²⁴ heavy metals,²⁵ gas molecules²⁶ etc. Recently, the union between GO, RGO and organic dye have carved a new formula for sensing Hg²⁺ ions. Graphene enters into noncovalent interactions with dye molecules forming supramolecular assemblies which can be fine tuned to sense various biological and chemical analytes. Our present study has its roots in a work reported by Zhang et al.²⁷ wherein they have revealed interesting interactions between G and various organic dyes.

Herein, we demonstrate the first use of a facile Hg²⁺ ion sensor based on reduced graphene oxide (RGO) and fluorescein (FLR) unit. Fluorescein is a typical synthetic organic dye noted for its unique spectroscopic characteristics. Selection of FLR as the target dye was done owing to its high extinction coefficient, low cost and due to its efficient interaction with RGO.^27–30 These merits are significant for fabricating a sensor based on GO, RGO and FLR. The interplay of competition dependant interaction of FLR and Hg²⁺ towards G sheets has paved the way for the selective detection of mercury ion. This typical sensor protocol relies on the decrease in absorbance/fluorescence intensity of the FLR dye on binding with G (turn off state) and the subsequent recovery of the absorbance/fluorescence intensity when in contact with the analyte ion (turn on state). Compared with other optical sensors, this proposed reduced graphene oxide-fluorescein unit (GFU) not only maintains high selectivity but also exhibits commendable limit of detection (780 ppt) with minimal demerits associated with common Hg²⁺ sensors. In addition to this, we have demonstrated the first use of GFU as a qualitative tool for easy monitoring of Hg²⁺. With the aid of GFU, ppm (parts per million) levels of Hg²⁺ ion can be discerned with visual colour change of GFU on adding Hg²⁺ ion. ppt level quantification of Hg²⁺ was attained by noting the colour change of GFU in presence of the former, when irradiated with UV rays. The highlight of our present study is the fabrication of a handy and user friendly fluorimetric strip of GFU which could detect ppb levels of Hg²⁺ in aqueous solution, which is of great significance as the prescribed limit of allowed Hg²⁺ in drinking water is 2 ppb.

Experimental

Materials and methods

All the chemicals used in this study were of analytical grade and were used without further purification. Graphite powder was purchased from across organics. Fluorescein dye (FLR) was obtained from Hi-Media. The stock solutions of FLR dye was prepared by dissolving known amount of FLR dye in deionised water. Different metal ions in the form of chlorides and nitrates (Hg²⁺, Cd²⁺, Pb²⁺, Cu²⁺, Fe³⁺, Zn²⁺, Na⁺, Ca²⁺, K⁺, Sr²⁺, Ba²⁺ and Ni²⁺) were prepared by dissolving required amount of their corresponding salts in deionised water (100 μM). Deionised water was used as the solvent throughout the investigation. pH of all the stock solutions was 8.0 ± 0.4, the normal pH of the solution. Tap water (from Calicut University, Kerala, India) was employed for real sample analysis.

The fluorescence emission spectra were done in Perkin Elmer Fluorescence spectrometer LS 55: excitation wavelength; 460 nm; slit width: 1 nm. The FTIR spectra of the samples were recorded on Jasco FTIR-4100 spectrometer using KBr disc method whereas the Raman spectra were taken on a LabRam HR-Horiba Jobinyvon Spectrometer using a Raman Microprobe with 532 nm Nd:YAG excitation source. A FEI TECNAI 30 G2, 300 kV were used to record the TEM images the sample. Absorption spectra of the solution were measured using Jasco V-550 spectrometer.

Synthesis of graphene

Graphene was synthesised by reduction of graphene oxide (GO) using hydrazine hydrate. Graphite oxide was prepared by Hummers method as reported elsewhere.³¹ About 100 mg of graphite oxide was dispersed in 100 mL of deionised water and the GO solution was treated with 104 μL of ammonia solution and 20 μL of hydrazine hydrate. The solution was then heated to 90 °C for 1 hour. During this process, reduction of GO occurs to yield graphene. The formed G solution was filtered and used for the studies. It needs to be mentioned that chemical reduction does not result in the formation of pristine G sheets; RGO is the actual product.

We have attempted to understand the interaction of GO and RGO with fluorescein. On comparing, amount of GO required to effect the reduction in absorption intensity by the same amount in presence of RGO is nearly ten times higher (about 83% reduction in absorbance was observed on addition of 110 μL of RGO, while 1.2 mL of GO was required to bring about the same effect). This suggests that the quenching ability is more prominent for RGO than that of GO. On chemical reduction, the ratio of sp² to sp³ domains group in RGO is higher than that of GO which increases the non covalent interactions between the dye and RGO sheets.^32,33 Hence, RGO was used as a quencher in this study.

Procedure for absorption and fluorescent studies

Stock solution of FLR (1 μM) and corresponding metal salts (100 μM) were prepared in DIW. Aliquots of RGO solution were added to 1 mL of FLR taken in a glass cuvette. Subsequently the change in absorption spectra was noted. Aliquots of metal ion solutions [Hg²⁺ (100 μM), all other metal ions at a concentration of 150 μM] were then titrated against GFU and the absorption spectra were recorded after every addition. The pH of the solution was 8.0 ± 0.4 during the whole study. At the concentration of Hg²⁺ selected for the present study (ppt and ppb levels), no mercury-hydroxide formation is noticed even at higher pH values (∼12). Hydroxide formation is detected only at mercury concentration > 120 ppm below this pH. Hence it is evident that the whole mercury is present as Hg²⁺ ions.^34–36

Fabrication of GFU as qualitative sensor for Hg²⁺

In order to evaluate the performance of GFU as a qualitative assay for Hg²⁺, we have adopted two optical channels. For colorimetric response, different metal ions (20 ppm) were added to GFU. (FLR: 1 μM, 1 mL & RGO: 20 μL) and the visual colour change was noted. For fluorimetric study, 780 ppt of various metal ion solutions were added to GFU (FLR: 1 μM, 1 mL & RGO: 20 μL) and the optical performance was viewed under UV light. Inorder to develop a solid phase sensor for Hg²⁺, GFU was immobilised on filter paper (Whatman 190 mm) by soaking in GFU solution for 12 h, followed by normal drying. Mercury ions were added on this paper strip and the fluorescence was monitored by placing the strip under UV illumination.

Results and discussions

The various physiochemical techniques adopted in the study confirmed the formation of reduced graphene oxide. The 26 nm shift of peak of GO towards the longer wavelength region on chemical reduction confirms the conversion of GO to RGO, as depicted in Fig. 1(a). The bands of GO at 230 nm arise due to π–π* of –C–C bonds of graphitic skeleton and –C=O of carboxylic acid group. In RGO, the absorption at 230 nm underwent a red shift to 256 nm implying the rejuvenation of electronic structure of RGO.³⁷

Fig. 1 (a) UV-Vis spectra of GO and RGO (b) FTIR spectra of GO and RGO, (c) typical Raman spectra of GO and RGO & (d) TEM image of RGO.

The typical FTIR spectra of GO and RGO are shown in Fig. 1(b). The spectra of GO present numerous bands pointing to the presence of various functional groups on GO. The bands at 1800 cm⁻¹, 1710 cm⁻¹ (–C=O stretching vibrations from carboxylic acid and carbonyl groups) and at 1100 cm⁻¹ (–C–O stretching vibrations) testifies the presence of oxygen containing functional groups. The strong bands at 3300 cm⁻¹ (–O–H stretching vibration) can be correlated to that of adsorbed water molecules. The peaks at 2979 cm⁻¹ and 2866 cm⁻¹ can be ascribed to the existence of –CH₂ and –CH groups in GO. In comparison to the FTIR spectra of GO, there is a drastic decrease in the intensity of IR bands in the FTIR spectra of R GO and hence the conversion is proved. The bands at 1710 cm⁻¹, 1707 cm⁻¹ and 1105 cm⁻¹ almost disappear in RGO.^38,39

Fig. 1(c) illustrates the Raman profile of GO and RGO. The two important bands namely the G band and D band arises due to the first order scattering of the E_2g phonon of sp² atoms and from the breathing mode of κ-phonons of A_1g symmetry. The I_D/I_G ratio quantifies the rate of disordered and ordered structure of RGO. The I_D/I_G ratio of G is 1.35 and that of GO is 1.2. This can be attributed to the formation of smaller sp² domains and due to the existence of unrepaired defects on reduction. This observation goes parallel to previous reports.^40,41

Fig. 1(d) depicts the TEM image of RGO. The TEM image of RGO sheets confirms the formation of multilayer RGO sheets on chemical reduction. It is relevant to mention here that it is almost improbable to obtain pristine G sheets via chemical reduction and the end product is actually reduced graphene oxide.

Optical response of reduced graphene oxide-fluorescein unit (GFU)

Interaction between Reduced Graphene Oxide (RGO) and fluorescein (FLR) was investigated by observing the changes in the absorption peak of FLR centered at 489 nm. On titrating RGO against FLR dye, there was a drastic reduction in the absorbance intensity of FLR as evident from the Fig. 2(a). This stage is realized as ‘turn-off’ state as there was a visible colour change from bright green to dark green with the addition of RGO (Fig. 2(b)). FLR will be adsorbed on the surface of RGO sheets via non covalent interactions to form a reversible supramolecular, but stable assembly.²⁷ The FLR chromophore being negatively charged at the experimental pH, will bind to RGO through π–π interaction and forms a supramolecular complex as described by the equation;

nRGO + nFLR ⇆ (RGO⋯FLR)_n.

Fig. 2 (a) Change in absorption spectra of FLR (1 μM, 1 mL) on addition of aliquots of RGO solution (110 μL) & (b) digital images of FLR dye and GFU.

As the titration progresses, the extent of adsorption of FLR onto RGO sheets increases leading to a decrease in the amount of free dye molecules.

We have also investigated the fluorimetric responses of the dye (1 μM, 1 mL) with the addition of aliquots of RGO (150 μL). Fig. 3(a) depicts the fluorescence emission profile of GFU. AS mentioned earlier, the formation of (RGO⋯FLR)_n supramolecular assembly leads to quenching of fluorescence of FLR at 516 nm (turn off state) via Fluorescence resonance energy transfer (FRET).⁴² The digital image of FLR and GFU in UV light clearly establishes the quenching of fluorescence of FLR in presence of RGO (Fig. 3(b)).

Fig. 3 (a) Fluorescence emission spectra of fluorescein dye (1 μM) on addition of RGO (200 μL) & (b) digital images of GFU (left) and FLR (right) in UV light.

Selectivity study

As a preliminary move, various ions (Hg²⁺, Cd²⁺, Pb²⁺, Cu²⁺, Zn²⁺, Na⁺, Ca²⁺, Fe³⁺, K⁺, Sr²⁺, Ba²⁺, Ni²⁺ at a concentration of 150 μM, 180 μL) have been screened for the affinity towards GFU. Remarkably, no ion except Hg²⁺ could affect significant restoration of the absorbance spectra of GFU (Fig. 4). The suppressed absorbance of GFU was almost restored to initial value on adding Hg²⁺ ions. Though the congeners of mercury i.e. Cd, Pb and Zn restored the suppressed absorbance intensity, the restored intensity was negligible (less than 2%) when compared to that effected by Hg²⁺. For interference testing, all metal ions (including and excluding Hg²⁺ ions) were mixed to form solution and were titrated against GFU. Mixed solution without Hg²⁺ failed to restore the suppressed absorbance value of GFU. Whereas the mixed solution with Hg²⁺ could affect restoration of the absorbance maxima. The above study assessed and established the selectivity of GFU towards Hg²⁺ ions over other common interfering metal ions. It is quite evident that the unit exhibited admirable selectivity towards Hg²⁺ in presence of other interfering ions, pointing the ability of the projected appliance to function in a complex environment.

Fig. 4 Absorption responses of GFU to some interfering ions. (Hg²⁺, Pb²⁺, Cd²⁺, Zn²⁺, Cu²⁺, K⁺, Fe³⁺, Na⁺, Sr²⁺, Ca²⁺, Ni⁺ & Ba²⁺). A_reg is regained absorbance of GFU on addition of metal ions – absorbance of GFU.

GFU as a quantitative colorimetric/fluorimetric sensor for Hg²⁺ ion

The sensing property of GFU was studied by colorimetric and fluorimetric channels. Fig. 5(a) reveals the ‘turn-on’ absorbance profile of GFU on addition of aliquots of Hg²⁺ ion. As the concentration of Hg²⁺ increased from 0 to 280 ppb, the decreased absorbance of GFU was restored to its initial state indicating that GFU can be employed for sensing Hg²⁺. A linear correlation existed between the absorbance intensity and the concentration of Hg²⁺ (Fig. 5(b), R² = 0.98). The GFU exhibited limit of detection (LOD) of 240 ppb as calculated by 3δ/slope rule and the projected unit can function as a colorimetric quantitative tool for Hg²⁺ determination in unknown samples. The LOD value exhibited by the unit was almost comparable or further lower than several of the colorimetric sensors reported previously. The GFU outweighs other reported colorimetric sensors as it is highly specific with a notable LOD value and it does not demand complex modification steps, masking/buffering agents.^43–47

Fig. 5 (a) UV-Vis spectra recorded during the titration of Hg²⁺ (20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, 260, 280 ppb) against GFU & (b) quantitative variation of absorbance of GFU with concentration of Hg²⁺ ion.

The fluorescence characteristics of GFU were greatly influenced by the presence of Hg²⁺. Fig. 6(a) presents the fluorescence restoration of GFU on addition of Hg²⁺. Complete restoration of quenched fluorescence intensity was achieved upon addition of ppt amount of Hg²⁺ ions (turn-on state). There was a linear variation (Fig. 6(b), R² = 0.9921) in fluorimetric response with the concentration of Hg²⁺ ions (0 ppt to 800 ppt) enabling the use of GFU as a quantitative probe for Hg²⁺ ion detection in unknown samples. Being a more sensitive technique, fluorimetry yielded a much low LOD value of 780 ppt, when compared to the colorimetric approach. This significantly low LOD value asserts the supremacy of GFU in mercury sensing when matched with previously reported Hg²⁺ fluorimetric chemo sensors. Notably, the LOD exhibited by the unit was much lesser than the LOD prescribed by European Union.^47–54 This significant low LOD value asserts the bright prospects of GFU in the heavy metal ion sensing scenario.

Fig. 6 (a) Fluorescence spectra of GFU in presence of different concentrations of Hg²⁺ (50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800 ppt) & (b) plot of fluorescence intensity versus Hg²⁺ concentration. Excitation wavelength: 460 nm, slit width: 1 nm.

Mercury ion (Hg²⁺) sensing strategy by reduced graphene oxide-fluorescein unit

This interesting optical behaviour of GFU in presence of Hg²⁺ prompted us to explore the mechanism behind this sensing. The probable sensing mechanism can occur via the displacement of fluorescein dye from the GFU with the entry of Hg²⁺ ion as a result of selective adsorption on RGO sheets. The principle underlying this sensing strategy relies on the bifunctional nature of RGO sheets to act as an absorbance/fluorescence nanoquencher and a highly specific nanosorbent. In the first stage, RGO binds with FLR dye by noncovalent forces forming a supramolecular assembly i.e. GFU. The noncovalent forces responsible for the formation of GFU is the (i) π–π interaction between the aromatic clouds of RGO and FLR; (ii) the dispersive interaction between negatively charged functional groups of RGO and the chromophore of FLR.²⁷ The evolution of GFU will pave the way for the suppression of absorbance and fluorescence intensity of FLR dye (turn-off state), as the amount of free FLR molecules in the solution decreases. When the strong competitor i.e. Hg²⁺ with higher affinity towards RGO sheets than FLR (weak competitor) is added to GFU, Hg²⁺ will be preferentially adsorbed over FLR by RGO. As FLR is gradually desorbed from the surface of the supramolecular assembly, the absorbance and fluorescence intensity will be restored. The π electrophilicity of Hg²⁺ towards the π cloud of RGO plays a pivotal role for the absorption of Hg²⁺ ion. RGO can selectively adsorb Hg²⁺ ion leading to the restoration of absorbance of fluorescein dye. It was found that there were no significant changes in the absorption spectra of fluorescein dye even after addition of 200 μL of Hg²⁺ ion, which rules out the possibility of interaction between the dye and metal ion.

To further elucidate the adsorption of Hg²⁺ ion by RGO sheets, ethylenediaminetetraacetic acid (EDTA), a typical complexing ligand (200 μM, 250 μL) for Hg²⁺ was added to the GFU/Hg²⁺ solution. As expected there was a decrease in absorbance maxima of fluorescein dye, since EDTA binds to Hg²⁺ ion leading to the reformation of GFU (Fig. 7(a)). The recognition process is reversible, as the subsequent addition of the complexing agent suppresses the absorbance characteristics.⁵⁵ The fluorescence study followed the same trend as the absorption study, i.e. the requenching of the fluorescence intensity of GFU on addition of complexing agent (Fig. 7(b)). Based on these results, the whole sensing process is schematically shown in Scheme 1.

Fig. 7 Elucidation of sensing mechanism by addition of EDTA (200 μM, 250 μL) recorded by (a) UV-Vis spectroscopy (b) fluorimetry.

Scheme 1 Schematic representation of the detection of Hg²⁺ ions by reduced graphene oxide-fluorescein complex. Introduction of Hg²⁺ ion restores the absorbance and fluorescence intensity of the unit.

As the proposed unit exhibited excellent selectivity and sensitivity in deionised water, the practicability and analytical capability of GFU were tested with tap water by spiking a known amount of Hg²⁺ ion into tap water (maintaining the concentration as 10 ppb). Fluorescence being the more sensitive technique, we opted it for the real sample analysis. Consequently, the fluorescence response of GFU was followed on addition of aliquots of tap water contaminated with mercury ion. It was demonstrated that the suppressed fluorescence intensity of GFU was restored on addition of tap water spiked with Hg²⁺ ion as shown in Fig. 8. The restoration of optical characteristics by fluorimetric techniques indicates the utility of the proposed GFU in real samples. It was quite soothing to note that the error% of the analysis was less than 1.5%.

Fig. 8 Restoration of fluorescence intensity by GFU in tap water contaminated with mercury ion (10 ppb).

GFU as a qualitative tool for Hg²⁺ ions

In this section, the extended application of the above-mentioned results is dealt with. In the first approach, the colorimetric channel was adopted for evaluating the sensing response of GFU towards various metal ions. Fig. 9(a) shows the impact of various ions on GFU in day light. There was a visible colour change in the GFU from dark green to pale green on addition of Hg²⁺ ion. No such observation was true for other ions selected (chosen randomly). Interestingly, an amount of Hg²⁺ at a concentration as low as 20 ppm could be detected using this moiety. This opens up a route for visual naked eye detection of the ion, as drastic colour variation was observed for the GFU solution in presence of the said ion. The observation is of immense importance from a layman's view as the allowed concentration of the ion in drinking water is 2 ppb.

Fig. 9 (a) Effect of various ions on GFU in visible light (metal ions (20 ppm), fluorescein dye (1 μM, 1 mL), RGO (20 μL)). (b) Effect of various metal ions (780 ppt) on GFU under UV light. Fluorescein dye (1 μM, 1 mL), RGO (20 μL). (c) Performance of GFU as a fluorimetric sensor strip for Hg²⁺. The fluorescence restoration was achieved with 20 ppb of Hg²⁺ ion under UV light.

It is already concluded that fluorimetric technique has lead to quantitative identification of Hg²⁺. Hence an attempt was made to qualitatively analyse the ion in presence of UV radiation. As clear from Fig. 9(b), GFU in presence of the test ion fluoresces in a greenish yellow colour while the other ions failed to exhibit the said fluorescence. Interestingly, ppt level of mercury ion, nearly 780 ppt itself was sufficient to cause the fluorescence. Based on this observation, a solid phase filter paper based sensor for Hg²⁺ was designed using GFU. Immobilisation of RGO-dye array on filter paper strip was achieved as explained in the experimental section. The restoration of greenish yellow emission was noted, which revealed the successful service of this solid phase sensor in ppb level detection of mercury ions Fig. 9(c). A minimum amount of 20 ppb of Hg²⁺ was required to restore the fluorescence of GFU adsorbed filter paper strips. The projected GFU filter paper strip serves as an elite and handy solid phase sensor for Hg²⁺ ion, which demands the mere presence of a UV source to selectively identify the latter. To the best of our knowledge, no report is there in the literature till date, which focuses on RGO-dye based solid phase Hg²⁺ sensors. Though some reports are there using other systems based on bis(ferrocenyl)azine, porphyrin, conjugated polymer etc.,^56–58 they fail in competing with the simplicity, efficiency, cost effectiveness and user friendliness of the system reported herein.

Conclusion

Summarising, we have demonstrated that GFU can effectively function as a Hg²⁺ ion sensor via colorimetric and fluorimetric approach. The bifunctional nature of graphene sheets to act as a selective adsorbent for Hg²⁺ and to suppress or quench the absorbance and fluorescence of fluorescein dye is well exploited in this study. In addition to this, the RGO-dye units stand as a real time handy sensor for Hg²⁺ ion with outstanding selectivity and sensitivity. The GFU exhibited an appealing value of limit of detection, viz.: 240 ppb and 780 ppt through colorimetric and fluorimetric route respectively. Moreover, GFU can serve as a qualitative sensing platform for Hg²⁺ ions enabling the naked eye detection of ppm levels of Hg²⁺ and visual detection of ppt levels of Hg²⁺ in presence of UV radiation. The excellent performance of GFU based solid phase sensor proposed in this study asserts the practical potential of the unit to detect ppb levels of Hg²⁺ ion in solution. Our investigation warrants the bright prospects of G dye units as an effective detection tool for the hazardous metal ion, Hg²⁺.

Conflict of interest

The authors declare no competing financial interests.

Acknowledgements

M. Anju gratefully acknowledges the financial assistance received from UGC and also University of Calicut for providing research facilities.

Notes and references

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