Microenvironment Sensitive Charge-Transfer Dye for Tandem Sensing

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Microenvironment Sensitive Charge-Transfer Dye for Tandem Sensing of Multiple Analytes at Mesoscopic Interfaces Nilanjan Dey, Dipen Biswakarma, Akash Gulyani, and Santanu Bhattacharya ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02065 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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ACS Sustainable Chemistry & Engineering

Microenvironment Sensitive Charge-Transfer Dye for Tandem Sensing of Multiple Analytes at Mesoscopic Interfaces Nilanjan Dey,[a] Dipen Biswakarma,[b] Akash Gulyani,*[a] and Santanu Bhattacharya*[b],[c] a

Institute for Stem Cell Biology & Regenerative Medicine, GKVK Post Bangalore 560065, b

c

Department of Organic Chemistry, Indian Institute of Science Bangalore 560012.

Director’s Research Unit, Indian Association for the Cultivation of Science, Kolkata 700032. Email: [email protected], [email protected]

KEYWORDS. Mesoscopic Interface, Microenvironment sensitive, Multiplexing, Tandem detection, Real-life samples.

ABSTRACT. An easy to synthesize amphiphilic dye is developed whose sensing behavior in surfactant assemblies can be modulated through surface-charge, micropolarity and local pH etc. Thus the micelle-bound probe shows remarkable ion-dependent bathochromic shifts in the charge transfer band, enabling simultaneous ratiometric detection of four different metal ions, such as Cu2+, Ni2+, Hg2+, and Zn2+ at parts per billion (ppb) level in water. This is indeed a striking observation, since naked-eye sensing of multiple metal ions at mesoscopic interface is

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not known till date. Moreover, the probe even shows distinct color response against copper ion in different oxidation states, which is also unheard of. Further, the in-situ formed metal complexes can be employed for naked-eye screening of three different amino acids, such as histidine, cysteine and aspartic acid in aqueous medium. Probes of this class, that are capable of multiplexing, offer new ways of efficiently screening multiple analytes in complex, real-life samples (eg. wastewater management, analysis of pharmaceutical drugs etc). Further, low-cost reusable dye coated-paper discs were also developed as an eco-friendly method for onsite sensing of metal ions.

INTRODUCTION Transition metal ions at recommended levels are essential for maintaining various biological processes in body, while alteration in their concentrations can cause several diseases. Conventional approaches of metal ion sensing, like atomic absorption, ICP-MS, electrochemical assays, gas chromatography, etc. are tedious processes involving multistep sample preparation and sophisticated analytical facilities. Thus recent years have seen a tremendous increase in the number of optical sensors for ionic analytes and biologically relevant neutral molecules due to their high sensitivity/selectivity, quick response time; low maintenance cost and easy detection procedure.1 However, the traditional optical probes mostly work on the specific one-to-one recognition strategy, where change in the electronic property of the receptor (recognition) site is transmitted to the covalently attached signalling moiety, leading to an optical signal. Over the years, researchers have also attempted to design molecules, capable of sensing multiple analytes simultaneously by producing distinguishable output signals.2,3 This kind of multiplexing probes can eventually reduce the time and cost associated with real-life sample analysis. However, the


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major challenge here is to ensure that the signal due to one target analyte should not be masked by the responses coming from other analytes. Though probes capable of detecting two analytes simultaneously are known in the literature, probes capable of sensing more than five analytes simultaneously are either rare or not available.4-6 Therefore, to sense multiple analytes simultaneously, one can either develop molecules with multiple binding sites or incorporate a group of sensory probes as an array.7,8 However, design of such multiplexing probes or arraybased systems needs either extensive multistep organic synthesis or complex engineering; along with rigorous standardization. Another possible way to achieve multiresponsive behavior is to modulate the probe microenvironment or add in target-specific guest entities. In these approaches, it may be possible to alter sensing ability of a probe molecule without modifying its chemical structure. Indeed, here we show multi-analyte sensing can be achieved through a careful modulation of the local microenvironment. The other pertaining issue to consider in sensing strategies is the availability of the target analyte. This is particularly relevant to metal ion sensing, since in biological media and living cells, metal ions are largely present in the bound forms, either in association with lipid membranes, proteins or other biomolecules. However, most of the efforts to sense metal ions have so far been confined to optical sensing in solution, either in organic solvent or bulk water. In order to investigate metal ion function and transport in the cellular environment, it is important to sense metal ions exclusively at bound state. This is a challenging task, since it needs the design of a probe that does not report the presence of metal ions in water, while being able to respond specifically to towards them in the bound state. Here we have addressed the dual challenge of multiplexing and metal ions sensing at complex interfaces. We show here the dramatic modulation of optical properties of a newly developed probe, 4-(2-pyren-1-yl-vinyl) pyridyl ketone (1) in micellar medium (Fig. 1). The electrostatic


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characteristics of the surfactant head groups, their intermolecular association and dynamics of palisade/stern layer markedly influence the physical properties of the embedded probes, including their acid-base equilibrium and metal ion sensing behavior. The color changing response of 1 (ratiometric probing) in presence of multiple metal ions, such as Cu2+, Ni2+, Hg2+, and Zn2+ ensures their effective detection with minimum background interference. Coordination of metal ions can inhibit isomerization and enhance the extent of intramolecular charge transfer. Both these effects together ensure unique ion-dependent shifts in the absorption maxima, which is indeed striking. Moreover, these in-situ generated metal complexes can be employed for selective recognition of three different amino acids (cysteine, histidine and aspartic acid) at parts per billion (ppb) levels in the aqueous medium. Considering these unique multi-stimuli responsive behavior, the present system was further explored to address real-life challenges, such as waste water management, pharmaceutical drug analysis etc.

Signaling unit

Metal ion Water Self-assembly No sensing

N

O

Binding site Probe-1 Surfactant

Metal ion Probe-surfactant assembly

Metal ion sensing

Figure 1. Schematic diagram shows exclusive metal ion sensing at mesoscopic interfaces EXPERIMENTAL SECTION


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Material and methods: All reagents and starting materials were obtained from the best known commercial sources and were used without further purification. FTIR spectra were recorded on a PerkinElmer FTIR Spectrum BX system. 1H and

13

C NMR spectra were recorded on a Bruker

Advance DRX 400 spectrometer operating at 400 and 100 MHz respectively. UV-visible and fluorescence Experiment: The UV−vis and fluorescence spectra were recorded on a Shimadzu model 2100 UV-vis spectrometer and Cary Eclipse spectrofluorimeter respectively. The micelle forming surfactant was weighed in a glass vial in appropriate quantity and solubilized with water. All the surfactants were taken at above critical micellar concentrations to ensure the formation of stable micelles. To obtain clear micellar solution, the mixture was stirred for 5 minutes and then sonicated ~2 min. The solution was kept at room temperature for 30 min to equilibrate and then high concentrated THF stock solution of probe was added. The co-solvent THF was present in very small amounts and did not affect the fluorescence behaviour. To study in aqueous medium, THF stock solution of 1 was directly added in water and stirred for 5 min. To monitored the effect of pH, sensing experiment were performed in buffered media of different pH (HCO2Na/ HCl buffer for pH 2, Tris/HCl for pH 7 and Na2B4O7·10H2O/NaOH for pH 12). Scanning Electron Microscopy: The samples were made under the dust-free condition and drop cast over double-sided tapes attached to the brass stubs. Then the stubs were air-dried for 48 h. The coatings with gold vapor were done before analyzing the samples on a Quanta 200 SEM operated at 15 kV. Fluorescence Decay Experiment: Fluorescence lifetime values were measured by using a time-correlated single photon counting fluorimeter (Horiba Jobin Yvon). The system was excited


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with nano LED of Horiba - Jobin Yvon with pulse duration of 1.2 ns. Average fluorescence lifetimes (τav) for the exponential iterative fitting were calculated from the decay times (τi) and the relative amplitudes (ai) using the relation, τav = (a1τ12+a2τ22+a3τ32)/(a1τ1+a2τ2+a3τ3)……………………………………………………..(1) Where a1, a2 and a3 are the relative amplitudes and τ1, τ2, and τ3 are the lifetime values, respectively. For data fitting, a DAS6 analysis software version 6.2 was used. Analysis of real-life samples Finely powdered tablet sample (Paracetamol), 20 mg was taken in a glass vial and mixed with 10 mL water. The mixture of stirred at room temperature for 2 h at 37 ºC to yield a clear solution. Then this solution was utilized for the preparation of anionic micelle (using SDS). To check whether the probe compound can detect Cu2+/Ni2+ presence of pharmaceutical drug (Paracetaml) or not, the SDS micelle laced with drug was spiked with different amounts of Cu2+ and Ni2+ and incubated for 2 h. To prepare the dye coated paper strips, filter paper was cut into circular pieces (diameter ~1 cm) and 40 mL MeOH solution of 1 was drop casted on to these filter paper discs using micropipette. The solution was completely absorbed in the filter paper within 15 min and became ready for use. RESULTS AND DISCUSSION Photophysical characterization of dye: In aqueous medium, compound 1 showed red-shifted absorption maximum (λmax = 430 nm) with no ‘pyrene like signature’ [highly-structured vibronic spectrum]. This indicates facile intramolecular charge transfer from the electron-rich pyrene to the electron-deficient pyridyl ketone unit via trans-olefinic bond (Fig. S1).9 Similarly, fluorescence spectrum of 1 in water as well as in different organic solvents showed significantly


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distinct emission signature compared to the parent pyrene molecule (Fig. 2a & S2).10 The width of half maximum of the emission band increased with an increase in solvent polarity, pointing to a 'polar’ excited state.11 However, the large Stokes shifts observed in protic solvents, like MeOH and EtOH, cannot be attributed to polarity alone. These shifts may also be caused by hydrogen bonding interaction of the protic solvents with pyridyl ketone unit in the excited state. Hydrogen bonding interaction is expected to stabilize the compound in the cis conformation, making it suitable for metal ion coordination.12 In water, along with H-bonding interaction, CT process is likely to be facilitated through self-aggregation as well. Dynamic light scattering studies in conjunction with transmission electron microscopy (TEM) further substantiate the notion of dyeaggregation in water medium (Fig. S3).

DMSO

600

THF

400 200 0 450

(b) 600

CHCl3

Toluene 500

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MeOH & H2O 600

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Fluorescence (a. u.)

(a) 800 Fluorescence (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 in Brij-58 1 in SDS

400

1 in CTAB 200

0 700 450

1 in water 500

Wavelength (nm)

550

600

650

700

Wavelength (nm)

Figure 2. (a) Emission spectra of 1 (10 µM, λex = 370 nm) in different organic solvents. (b) Emission spectra of 1 (10 µM, λex = 370 nm) in different micelle medium and water.

Interaction of 1 with metal ions: need of surfactant? The interaction of 1 with various metal ions was examined in water as pyridyl ketone unit is known for its metal ion chelating property. However, the addition of metal ions to 1 in water showed no or little change in dye absorption (Fig. S4). This is indeed noteworthy. It appears that due to the aggregation of probe molecules in water, the ion binding pyridyl ketone unit is in the buried state and remains inaccessible towards metal ions.13 Aggregation of 1 in water and its lack of sensitivity towards metal ions provide an


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opportunity of using this probe for context-dependent sensing of metal ions. It is welldocumented in the literature that amphiphilic surface/interface, created by the micellar solution of surfactants in water, can influence the aggregation behavior of dyes. Moreover, low apparent micropolarity inside the micellar core can enhance the possibility of noncovalent interactions, like hydrogen bonding, charge pairing etc.14 To support of these arguments, fluorescence spectra of 1 were recorded in water as well as in different micelle medium. In surfactant assemblies, the probe showed blue shifts in emission maxima as compared to that observed in water along with prominent enhancement in the intensity (Fig. 2b). This clearly indicates that the probe molecules experience more apolar environment once solubilized in the micelle, as compared to the bulk water medium.15-16 However, among various surfactant systems, red-shift in absorption, as well as emission maxima, was observed when the probe was incorporated in micelles with an anionic surface (Fig. S5). A similar kind of red shift was witnessed in the acidic environment (at pH < 4.5) (Fig. S6), which indicates that red-shift, might be occurring due to alteration in the acid-base equilibrium. On the other hand, micropolarity, as indicated by ET values, indicated that the probe experiences a relatively polar microenvironment in SDS (ET = 55.4) medium compared to Brij58 (ET = 42.5) and CTAB (ET = 40.2) (Fig. S7). Thus, we can conclude that the unique spectral signature observed in the anionic micelles is likely to be contributed by both lowering of local pH and polarity effect. In anionic micelles, the pyridine end gets protonated and becomes more electron deficient, which eventually facilitates the intramolecular CT interaction (Fig. S8).17 Moreover, the distinct fluorescence spectra of 1 in micelle medium and bulk water indicate differences in their arrangement as well as microenvironment. In surfactant assemblies, the compound can align itself in a way that hydrophobic pyrene part remains embedded in the hydrophobic cavity, while pyridyl ketone unit gets localized at the surfactant/water interface.


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1 in SDS (a)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) 0.20

+Hg2+/Zn2+

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+Ni2+

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+Cu2+

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Absorbance

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0.03 0.00 350

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0.15

1 in Brij-58

0.10

1+Hg2+

0.05 0.00 350

Wavelength (nm)

400

450

500

550

600

650

Wavelength (nm)

Figure 3. (a) Absorption spectra of 1 (10 µM) in presence of different metal ions (10 µM) in SDS (8 mM). (b) Absorption spectra of 1 (10 µM) in presence of different metal ions (10 µM) in Brij-58 (1 mM).

Interaction with metal ions in micelle medium Ion-dependent bathochromic shifts in CT band: Since micelles were able to ‘modify’ the nature of aggregates in water, we were interested to explore the effect of metal ions on the probe bound to micelles. In SDS micelle medium, the probe showed remarkable discrimination ability by exhibiting different extents of bathochromic shifts in CT band: 120 nm for Cu2+, 70 nm for Ni2+, 48 nm for Hg2+ and 40 nm for Zn2+ (Fig. 3a). Thus, we observed that the bright yellowcolored probe (1) solution becomes red, orange and deep yellow upon addition of Cu2+, Ni2+ and Hg2+/ Zn2+ respectively. This kind of ion-dependent red-shift in the visible region allows nakedeye detection of multiple metal ions. However, the compound was found to be silent when other divalent as well as trivalent transition metal ions or lanthanides were added under similar condition (Fig. S9). Titrations of 1 with the aforementioned metal ions invariably show saturations in optical signals upon addition of ~1 equiv. of M2+ (Fig. 4). Most importantly, in all cases, we could observe ratiometric variations in absorbance values with concentrations of added metal ions (Fig. S10). Also, the compound showed detectability of metal ions as low as ppb level in SDS medium (Table S1). The metal ion-induced bathochromic shifts in the absorption maxima can be rationalized in terms of enhancement in extent of intramolecular charge transfer


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(ICT) interaction. The coordination of a metal ion to the pyridyl ketone moiety improves its electron-withdrawing characteristics, which leads to a facile charge transfer from the electronrich pyrene unit to the metal ion-laced pyridyl ketone group. (a) 0.15

(b) 0.15 Absorbance

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Absorbance

With Hg2+

Absorbance

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With Ni2+

0.10

0.05

0.00 300

Wavelength (nm)

400

500

600

700

Wavelength (nm) 2+

Figure 4. UV-visible titrations of 1 (10 µM) with (a) Hg (0-8 µM), (b) Zn2+ (0-8 µM), (c) Cu2+ (0-8 µM) and (d) Ni2+ (0-8 µM) in SDS (8 mM).

The metal ion-specific red shifts in ICT band can be explained on the basis of charge density on the bound metal ion. Apart from this, ionic radius of the metal ions (hydrated) in water, geometries of their coordination complexes, crystal field stabilization energies individually or together can also control the energetic of charge transfer interactions (position of CT bands).18 Under long UV lamp, the probe showed intense orange luminescence, as evidenced by the formation of a fluorescence band at ~580 nm (Φ= 0.22). Among various transition metal ions, only Cu2+ and Ni2+ ions could quench the emission but to different extents (Fig. S11). As


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expected, titration of the compound with Cu2+ showed slightly higher Stern-Volmer constant value, compared to what observed with Ni2+ (Fig. S12 & S13). Differentiation of metal ions based on their redox-states: The decisive role of Cu+ in Menkes’ kinky hair syndrome, Alzheimer’s disease, or Wilson diseases is already known in the literature. However, detection of Cu+ is difficult as it can readily disproportionate into Cu2+ and Cu0 in water.19 Thus the number of sensor molecules reported for Cu+ is extremely less.20-23 Surprisingly, here we found that compound 1 could show very distinctive absorption spectra in presence of Cu2+ and Cu+ in SDS medium. Unlike Cu2+, Addition of Cu+ (added inform of Cu(PPh3)3Br salt) induced formation of orange-colored solution with ~45 nm red shift in ICT band (Fig. S14). Titration with Cu+ showed linear ratiometric variation in absorbance values with multiple isosbestic points. Compound showed 1:1 interaction with Cu+ with binding constant 3.95 ± 0.02 x 106 M-1 (Fig. S15). Most importantly, in presence of strong chelating agents, like neocuproine, we observed reversal of optical signal. This indicates that compound 1 is the truesensor for Cu+ as well (Fig. 5a). The difference in the optical behavior might be coming from a diverse mode of interactions; Cu2+ is known to form paramagnetic square planar complexes, while Cu+ prefers diamagnetic tetrahedral complexes. 1H-NMR spectrum of 1 in DMSO-d6/D2O mixture medium with Cu(PPh3)3Br did not induce paramagnetic quenching, which supports the above conjecture (Fig. S16). Formation of 1:1 complex with Cu+ was also evidenced by ESI-MS mass spectral analysis (Fig. S17). To confirm the selective detection of Cu+ and Cu2+, the control experiments were performed in presence of excess of oxidizing as well as reducing agents. In presence of the oxidizing agent (H2O2), the addition of Cu+ led to a similar spectral change as was obtained with Cu2+ (Fig. 5b). Similarly, reverse situation was obtained when Cu2+ was added in presence of reducing agent (glutathione). Therefore, the probe could not only differentiate


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copper ions at two different oxidation states, it can also detect the in-situ generated Cu+ or Cu2+ ion during physiological transformations.

(a) 0.20

(b) 0.20 blank + Cu+ 0.16 + Neocuproine 0.12 + Cu2+

0.16 0.12

+ Neocuproine

0.08

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Absorbance

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+ Cu2+ + GSH + Cu+ + Cu+ + H2O2

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Figure 5. (a) Absorption spectra of 1 (10 µM) in presence of Cu2+ and Cu+ (10 µM) in SDS (8 mM) with chelating agents. (b) Sensing of in-situ formed Cu2+ and Cu+ on absorption spectra of 1 (10 µM) in SDS (8 mM).

Insight into metal ion binding process: Stoichiometry of interaction between the sensor molecule and metal ions were determined by continuous variation method keeping their total concentration fixed. In all the cases, irrespective of positions of the CT band, we observed formation of 1:1 complex with metal ions, which was also evident by ESI-MS mass spectral analysis (Fig. S18). The binding constant values derived from Benesi-Hildebrand method show wide variations (Table S1). The binding affinity with Cu2+ was found to be higher as compared to Ni2+. This observation was in the good agreement with the Irving–Williams order.24 Most importantly, the optical changes with metal ions (Cu2+, Ni2+, Hg2+ etc) were found to be reversible in nature. Addition of proper chelating agent could completely reverse the spectral signature (Fig. S19). To further elucidate the structures of metal ion-bound 1, FT-IR and NMR spectroscopic techniques were employed. In presence of metal ions, the FT-IR peaks corresponding to the carbonyl and imine group of the pyridyl unit showed marked shifts in stretching frequency values toward the lower energy region (Fig. 6a & Table S3). These IR data


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confirms that both the pyridyl nitrogen and carbonyl unit of the pyridyl ketone moiety are involved in the metal coordination. (a)

(b)

300

+ Cu2+ + Ni 2+

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+ Hg2+

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O N

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b

c

d

e

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X

Figure 6. (a) FT-IR spectra of 1 with different metal ions in SDS.(b) Time-dependent emission spectra of 1 (10 µM, λex = 370 nm) in presence of metal ions in SDS (8 mM). (c) Struture of compound 1 and 1+M2+. (d) 1H-NMR titration of 1 (5 mM) with different metal ions in DMSO-d6/D2O (5:1) medium.

Time-dependent emission spectra showed that both in presence and absence of metal ions, compound 1 in SDS micelle showed multi-exponential decay pathways. However, decrease in average lifetime values was observed in presence of Cu2+ and Ni2+ (Fig. 6b & Table S2). Because of the high paramagnetic nature of Cu2+ and Ni2+, almost no residual peaks were observed in 1H-NMR spectrum of 1 with these metal ions. On the other hand, 1H-NMR spectrum of 1 with Zn2+ and Hg2+ indicates that the protons associated with ‘pyridyl ketone’ moiety experienced larger downfield shifts as compared to the pyrenyl protons. This undoubtedly proves the participation of the pyridyl ketone group in the metal ions binding process (Fig. 6c). Most


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importantly, the extent of downfield shifts was found to be dictated by the nature of the metal ions, more precisely on their influence on CT interaction (Fig. 6d). To elucidate the indispensable role of both pyridine nitrogen and carbonyl unit in the formation of metal ion recognition cleft, we have incorporated a reference dye (2) in studies, where we have phenyl group in place of pyridine end. As expected, this control compound showed no perceptible interaction with any of these metal ions (Fig. S20). Role of surface charge on metal ion sensing behavior: The sensing ability of 1 showed a clear dependence on the nature of the micellar interface/aggregate. In SDS, the probe experienced relatively polar surroundings with slightly acidic local pH. Thus, we expect that might be these changes in microenvironment will affect the sensing behavior of the reporter dye. Unlike the previous case, 1 in Brij-58 micelle medium, showed interaction solely with Hg2+ (Fig. 3b & S21). Addition of Hg2+ induced formation of the red-colored solution with ~70 nm red-shift in CT band. Though a significant loss in sensitivity was witnessed in this medium, retention of well-defined isosbestic points during titration studies indicates ratiometric detection of Hg2+ (Fig. S22). Moreover, in Brij-58 medium, Hg2+-induced a ~6-fold quenching of yellow-colored emission (λmax = 550 nm) probably due to heavy-metal ion effect (Fig. S23 & S24).25 Though Hg2+ showed the formation of 1:1 complex with 1 in Brij-58 medium, the binding affinity was found to be significantly lower compared to that observed in SDS (Fig. 8a & S25). On the other hand, no significant change in absorbance was observed when metal ions were added to 1 in CTAB micelle. The positively charged head group of CTAB surfactant may restrict the approach of metal ions towards the binding site. From the above shred of observations, it was quite evident that the selectivity as well as sensitivity of the probe molecule towards target metal ions largely depends on the physicochemical characteristics of the surfactant assembly. Since in SDS


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medium, the probe compound resides near the micelle-water interface, they will largely be available for target analytes. Further, this accessibility gets better due to the electrostatic association between the negatively charged head group of SDS (-SO3-) and the incoming metal ions. In addition, surfactant assemblies also control the mode of metal ion interaction with the sensor molecule. When metal ions were added to the probe solution in SDS, they need to compete with the proton to attach with the pyridine’s nitrogen, whereas in Brij-58 medium, the metal ions can directly interact with pyridyl ketone unit (Fig. 7). O O

H+ N

N

H

H+ H+ In Brij-58

In SDS

+ Hg2+

+ Hg2+

+ M2+ = Cu2+, Hg2+, Ni2+, Zn2+

H+

+ Hg2+

+ Ni2+

Hg2+

M2+

+ Hg2+

Hg2+

M2+

+ Zn2+

Hg2+

+ Cu2+

M2+

Figure 7. The schematic diagram shows multiple analyte sensing on the mesoscopic interface using probe 1 in water.

Ion-specific microstructure formation: Most importantly, metal ions also showed diverse impact on the microstructure formation of 1 in micelle medium. Scanning electron microscopic images of the compound in various micelles showed distinct morphologies. Unlike Brij-58 and CTAB medium, the formation of a leaf-like structure was noticed in SDS (Fig. S26). However, in presence of Hg2+ and Zn2+, we witnessed formations of ‘truncated fibril’ structure. On the other hand, the addition of Cu2+ and Ni2+ indicated clear disruptions in well-defined febrile


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morphology (ill-defined structures). Though we can identify the presence of some spherical nanoaggregates in presence of Ni2+, no such observation could be made with Cu2+ (Fig. 8b). Thus, microstructure formations of 1 in SDS medium are clearly governed by the nature of the added metal ions. However, this unique observation needs further investigation, which is beyond

(a)

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scope of this manuscript. Abs. Ratio in SDS medium

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(b)

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1 + Ni2+

1 + Cu2+

Figure 8. (a) Ratiometric variation in absorbance value of 1 (10 µM) upon addition of Hg2+ in Brij-58 (1 mM)and SDS medium (8 mM). (b) SEM images of 1 in presence of different metal ions (1:1).

Interaction with anionic analytes: As the natural water samples may contain different anionic analytes in addition to metal ions, we have checked interaction of compound 1 with naturally abundant anions, such as halides, phosphates, sulphates, carbonate etc. both in SDS and Brij-58 medium (Fig. S27). However, no interaction was observed when these anions were added as competitive analytes. Further, we studied the effect of anions on the sensing of Cu2+, Hg2+, Zn2+ and Ni2+ ion in SDS medium. Though few anions, like S2- or PO43- showed little interference in Cu2+ and Zn2+ ion detection, their influences were too low to be considered (Fig. S28). The negatively charged head-groups of SDS micelle probably repel the incoming anions and ensure the interference-free sensing of metal ions. To confirm this point further, we also followed the interaction of 1 with Hg2+ in neutral micelle (Brij-58) in presence of these anions. As expected,


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here we could witness larger influence of anions (particularly S2-) than that observed in SDS medium (Fig. S29). The effect of glutathione on the metal ion sensing was also investigated considering its high affinity towards transition metal ions. Though addition of GSH showed no impact on Cu2+ or Ni2+ sensing, slight intervention was observed with Zn2+ and Hg2+ ions (due to competitive binding) (Fig. S30). Multiple amino acids sensing in micelle medium Considering the reversible nature of the metal ion coordination, the in-situ formed metal complexes were further employed for tandem sensing of amino acids. Color changing response (ratiometric) with amino acids is purely governed by their relative affinities towards pre-added metal ions. The compound alone (without M2+) showed no interaction with amino acids both in Brij-58 and SDS medium (Fig. S31-S32). However, in presence of Cu2+, the probe interacted both with cysteine and histidine in SDS medium (Fig. 9a). In both the cases, the color of the solution changes from deep red to bright yellow. Titration studies showed ratiometric responses in both the cases, which indicate interference-free naked eye detection of amino acids in the aqueous medium (Fig. S33-S34). Similarly, amino acids checking in presence of Ni2+ showed selective interaction with histidine with a change in visible color from orange to bright yellow (Fig. 9b & S35). Thus we can selectively detect both cysteine and histidine by varying the metal ion template. On the other hand, in-situ formed 1+ Zn2+ complex showed interaction with both aspartic acid and cysteine (Fig. 9d & S36-S37). Since interaction with cysteine was also observed with 1+ Hg2+, we can easily detect both these amino acids individually without any problem (Fig. 9c & S38). Thus, by and large, we can detect three different amino acids (cysteine, histidine and aspartic acid) using a single probe compound.


17


As additions of Cu2+ and Ni2+ diminished the fluorescence intensity of probe, a recovery of emission signal was observed both in presence of cysteine (with both Cu2+ and Ni2+) and histidine (with Ni2+) (Fig. S39-S40). This is really an interesting observation, as the turn-on responses were observed at longer wavelength region (~580 and ~550 nm), the system can suitably be employed for multicolor bioimaging studies. The absorption spectra of 1 + M2+ in presence of target amino acids show uncanny resemblance with absorption spectra of 1 in micelle medium (Fig. S41). This certainly indicates the dissociation of metal ion-dye conjugates in presence of amino acids, which was also substantiated by EDTA-mediated recovery studies (Fig. S42). Again, here also the detection limits were found in the ppb level (Table S4).

2+

W ith Cu

With His

0.04

Blank 0.02

With Cys

(b) 0.04 A-A0 at 530 nm

Blank

0.01

2+

+1 +2 +3 +4 +5 +6 +7 +8 + +19 0 +1 +11 +12 3 +1 +14 5 +1 6

C u

0.02

1+

(c) 0.05

(d) With Cys

A-A0 at 500 nm

W ith Hg2+

0.03 0.02

With His

0.03

0.00

0.00

0.04

With Ni2+

1+ N 2 i + +1 +2 +3 +4 +5 +6 +7 +8 + +19 +10 +11 +12 +13 +14 +15 6

A-A0 at 530 nm

(a) 0.06

A-A0 at 500 nm

Blank

0.08

With Zn2+

0.06 0.04

With Asp

With Cys

Blank

0.02

0.01

+1 +2 +3 +4 +5 +6 +7 +8 +9 +1 0 +1 1 +1 2 +1 3 +1 4 +1 5 +1 6

2+

H g

1+ Zn 2 + +1 +2 +3 +4 +5 +6 +7 +8 + +19 +10 +11 +12 +13 +14 +15 6

0.00

0.00

1+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 30

Figure 9. (a) Change in absorbance of 1+ M2+ (1:1, [1] = [M2+] = 10 µM) upon addition of different amino acids (3 equiv.) in SDS medium (8 mM). (a) with 1+ Cu2+, (b) with 1+ Ni2+, (c) with 1+ Hg2+, (d) with 1+ Ni2+. (from left to right: Ala, Arg, Gly, Phe, Asp, His, Glu, Ser, Thr, Pro, Cys, Val, Iso, Leu, Tyro, Tryp, 1-16).

Density functional calculation of 1 using B3LYP/6-31G* level of theory showed pyrene as a distinctively electron-rich region, while electron density was quite low at pyridyl ketone unit


18

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(Fig. S43b).26 FMO analysis indicated that the HOMO of the compound was mostly concentrated on the electron-rich pyrene unit, while LUMO focused on electron-deficient pyridyl ketone moiety. Thus, it is quite obvious that the intramolecular charge transfer will occur from pyrene to pyridyl ketone (Fig. S43a). Addition of metal ions showed significant increase in molecular dipole moments of 1, which indicates facile intramolecular charge transfer with formation of redshifted absorption maxima (Fig. S43c). Moreover, complexation with metal ions showed decrease in the optical band gaps, where the gap depends on the nature of the metal ion. The energy gap was found to be lowest for Cu2+, followed by Ni2+, Hg2+/Zn2+ accordingly (Fig. S44). Application to real-life samples analysis: wastewater management. Though metal ions play essential roles in the proper functioning of several physiological processes, their excess intake may cause severe health problems, including death. For example, presence of excess Cu2+ in body can cause Wilson disease, Alzheimer's disease etc.27 Exposure to excess Ni2+ can result in allergic contact dermatitis,28 while Hg2+ poisoning showed long-term detrimental effect in body, including infamous Minamata disease.29 Thus it is important to devise reliable, low-cost method to check the presence of these metal ions in drinking water samples, particularly beyond their permissible levels. Considering this, here we have collected water samples from three different sources, such as tap, pool and Arabian sea and determined presence of toxic metal ions, such as Cu2+, Ni2+ and Hg2+ using the current colorimetric method. In all the cases, reliability was verified using traditional atomic absorption spectroscopic (AAS) technique (Table S5-S7). In most of the cases, the recovery values varied from 93% to 105% with relative standard deviations (RSDs) in the range of 1.1 – 4.8%. Small standard deviation values indicate a high accuracy of the present method. Most importantly, here instead of incorporating three different


19


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dye molecules, we can achieve semi-quantitative estimation of three different metal ions using a single molecular probe. Blank Hg2+ Zn2+ Ni2+ Cu2+

Ni2+

+ Cysteine

1-Cys

Masking of Zn2+ Cu2+ Hg2+

N

O

=

M2+

1-His

Hg2+ Zn2+ Ni2+ Cu2+

Hg2+ Zn2+

+ Histidine Masking of Ni2+ Cu2+ Cu2+ Hg2+ Ni2+

1-Asp

N O

+ Aspartic acid

Metal ion sensing

Masking of Zn2+

Figure 10. Exclusive detection of metal ions in SDS medium using amino acids as masking agent.

However, the real-life samples (particularly obtained from natural resources) often contain more than one metal ions. Thus, one needs to develop a method ‘smart’ enough to selectively detect each of these metal ions even in the soup of others. Compound 1 in presence of a mixture of competitive metal ions, such as Cu2+, Hg2+, Zn2+ and Ni2+ showed color change similar to 1 + Cu2+ system. Thus, the presence of other metal ions, except Cu2+ remains untraceable. Therefore, to detect other metal ions as well, we have employed amino acids as suitable ‘masking agents’.30,31 By exploiting the preferential affinity of amino acids towards metal ions, we can specifically detect one or two metal ions even in the mixture of others. For example, Ni2+ can be selectively silenced, if we used 1+Histidine as the ‘reactive agent’ instead of 1. Similarly, in presence of cysteine, three different metal ions, like Cu2+, Hg2+, Zn2+ will be in the complexed states. So, only Ni2+ will be available for interaction with 1. Thus, by comparing the signals obtained in presence of various probe + amino acid mixtures, such as 1+cysteine (reagent-1),


20

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1+histidine (reagent-2), 1+ aspartic acid (reagent-3), we can selectively identify Hg2+, Zn2+ and Ni2+ even in the mixture (Fig. 10). Application to real-life samples analysis: paper strips for on-site detection. Laboratorydeveloped methods for metal ion sensing mostly involve sophisticated analytical tools, which limit their applicability in real-life sample analysis. Thus, along with conventional studies, herein we have devised a more sustainable strategy of rapid, on-site detection of toxic metal ions using low-cost, reusable paper discs.32,33 The dye-coated paper discs appeared yellow in color under daylight with bright orange-colored emission. However, changes in color from yellow to violet and red were observed respectively when Cu2+ and Ni2+-contained aqueous solutions were spiked on these paper discs. Moreover, both of these metal ions could efficiently quench the native orange-colored emission of the discs. Though Hg2+ and Ni2+ also resulted in the change of color from yellow to orange, no significant alteration was observed in emission color (Fig. S45). The changes in emission color were further quantified by ImageJ software (Fig. S46).34 As the sensing using paper-strips do not require any expensive visualizing tool, this method will be suitable for on-site detection at open markets or other distant locations (rural areas). The price of each strip will be very less and can even be recycled upon washing with EDTA solution (Fig. 11a). 1

+ Cu2+ + EDTA + Ni2+ + EDTA

(b)

1 alone

(a)

F/F0 at 580 nm

1.0 1.0 0.8 0.6 0.4 0.2

+Cu2+

0.8

+Ni2+

0.6 0.4 With Cu2+ With Ni2+

0.2 0.0 0

1

2

3

4

5

6

Metal Ions added (in µM)

+E D

TA

2+

i +N

TA +E D

+C u

2+

0.0 Bl an k

Intensity of orange color

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



21


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Figure 11. (a) Reversible metal ion sensing using compound 1 coated paper strips (observed under UV lamp), quantified by ImageJ software. (b) Change in emission intensity of 1 upon spiking with Cu2+ and Ni2+ in paracetamol laced (2 mg/mL) SDS micelle medium (8 mM)

Application to real-life samples analysis: Impurity in pharmaceutical drugs. Synthetic procedures involving transition metal catalysis is very common in pharmaceutical drug designing.35 Metal ions, such as Cu2+, Ni2+, Pd2+, Cr3+ are routinely used in a large number of organic reactions, including the hydrolysis of carboxylic acid esters/amides, Schiff bases, carboxylation and decarboxylation reactions, hydrogenation, and nucleophile displacement reactions etc.36 Thus traces of these metals to remain in the pharmaceutical drugs even after purification is one of the major concerns of manufacturing industries. However, presently available regulator systems for impurity profiling of pharmaceutical products are mostly outmoded. Keeping this in mind, here we have considered Paracetamol as a model drug and estimated the presence of toxic metal ions, such as Cu2+ and Ni2+ in its aqueous extract. Addition of these metal ions (0 – 5 µM) to the diluted aqueous extracts of tablet showed concentrationdependent linear changes in the emission intensity at 580 nm band (Fig. 11b & S47). Further, the amounts of Cu2+ and Ni2+ in the spiked samples were determined using the regressive equations, Y = 0.995-0.177x (r2 = 0.999)and Y = 0.992-0.963x respectively (Fig. S48). The relative standard deviation values for Cu2+ varied from 1.4 – 5.1%, whereas for Ni2+, they are found in the range of 0.73 – 4.7% (Table S8-S9). CONCLUSIONS In conclusion, we explored photophysical property of a relatively simple, donor-acceptor based bifunctional compound, 1 in the aqueous medium. The compound in water showed evidence of pronounced self-aggregation and thus remains silence towards transition metal ions. Moreover, it also showed microenvironment-sensitive modulation in photophysical properties, when doped


22

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into surfactant assemblies. Micelle bound 1 showed unique ion-dependent bathochromic shifts in charge transfer band. Thus we can detect four different metal ions by ratiometric color change, Cu2+ (red color), Ni2+ (orange color) and Hg2+/ Zn2+ (deep yellow). This is the first report of naked-eye sensing of multiple metal ions in micelle medium. Interestingly, the present method also provide an easy way to discriminate between Cu2+ and Cu+ at mesoscopic interface, which is also unheard of till date. The reversible coordination of the probe molecules with metal ions is further explored for the screening of amino acids. Here, we can achieve selective sensing of as many as three different amino acids, such as cysteine, histidine and aspartic acid. Thus this unique probe eventually serves both the purpose, multiplexing as well as detection of analytes in the bound form. Further, the present stategy was employed to engeener a sustainable method for trace level detection of toxic metal ions in waste water samples and pharmaceutical drug extracts. Low-cost resuable paper strips caoted with sensor material were developed parallely for on-site detcetion purpose (without involving any sophisticated instrument). We hope that development of such multitasking probes will eventually reduce the time and cost associated with real-life sample analysis. ASSOCIATED CONTENT Supporting Information. contains additional UV-visible spectra, real-life sample analysis, characterization data (1H-NMR, ESI-MS, and DLS etc.) ACKNOWLEDGMENT SB thanks DST (J. C. Bose Fellowship) for the financial support of this work. AG and ND thank Institute for Stem Cell Biology & Regenerative Medicine and DBT, India for current financial help. SB and DB thank Indian Institute of Science for research fellowship. SB also


23


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thanks Indian Association for the Cultivation of Science, Kolkata for the financial support of this work presented in this manuscript. REFERENCES 1. Tadepalli, S.; Slocik, J. M.; Gupta, M. K.; Naik, R. R.; Singamaneni, S. Bio-Optics and BioInspired

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Low-cost colorimetric Probe for simultaneous detection of multiple analysis: An efficient strategy for wastewater management 229x134mm (96 x 96 DPI)


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Microenvironment Sensitive Charge-Transfer Dye for Tandem Sensing

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