Highly Efficient Chemical Sensors For Visual Detection And Quantification Of Toxic Pollutants In Aqueous Solution

The quest for industrial and biotechnological revolution has been contributed in increasing environmental contamination issues,worldwide.The controlled or uncontrolled release of hazardous pollutants from various industrial sectors is one of the key problems facing humanity.Among them,adverse influences of heavy metals,hydrazine,azo dyes,phenols,TMB and other various emerging pollutants on human health are well known to cause many disorders like reproductive,neurological,endocrine system,and cardiovascular,etc.Besides their presence at lower concentrations,most of these toxic pollutants are posing noteworthy toxicological concerns.In this context,various environmental regulatory authorities are contributing in the monitoring of the increased concentration of theses toxic pollutants.A number of traditional methodologies including,electrochemical sensors,inductively coupled plasma atomic emission spectroscopy,cold atomic absorption spectroscopy,and inductively coupled plasma mass spectrometry are used for the purpose.No doubt,these methodologies can detect these toxicants with great efficiency.However,there are certain drawbacks such as;they are expensive,time intensive,laborious,complex,limit of hiring skilled individuals etc.limit their usefulness.Therefore,real-time monitoring is urgently required.This necessitates the exploration for novel and efficient methodologies for recognition of these toxic agents.Among various methodologies adopted for tailoring such constructs,generally the methodologies,in which changes associated with spectral properties and visual detection,are preferred for the deceptive ease in the recognition process.Accordingly,a promising modality has emerged in the form of colorimetric monitoring of these toxic agents.Rhodamine and its derivatives proves to be excellent candidate for the purpose.However,most of the rhodamine-based sensors only operate in organic solvents or in organic/aqueous mixed solution,where they show very good properties like good sensitivity,selectivity and low LOD（nM）.Although there are some limited reports on rhodamine-based probes worked in aqueous solution for these toxicants detection,their properties are not satisfactory,and the LOD of them is generally in the order ofμM.To,solve these issues we have tried the polymer based vesicular sensors.The advantages of vesicular chemosensors lie in several aspects.Firstly,it can be operated in water efficiently due to the good aqueous solubility and stability of polymer vesicles.Secondly,the introduction of probes into vesicles can enrich the concentration of them in a local volume,which can greatly enhance the detection sensitivity and selectivity of probes.Thirdly,the polymer vesicles are size controllable and stable,and can be facile functionalized to get designable properties.However,in spite of great advancement in the field of polymer vesicle sensors for the recognition of biologically important species,little attention has been paid for the detection of such hazardous pollutants from industrial waste.Herein,we investigated the aqueous fluorescent sensor based models and their potentialities to address the detection fate of hazardous pollutants for a cleaner environment.1.Novel rhodamine-typed chemosensor for the detection of Hg²⁺and Cr³⁺ions in solution and inside cellsRhodamine and its derivatives have excellent photophysical properties and supposed to be highly efficient system for the visual detection of toxic heavy metals.Combined with the small organic molecule containing highly electronegative element（N,O and S etc.）these molecules can be used for the visual detection of Hg²⁺and Cr³⁺.Herein,we developed a rhodamine-2-amino-5-bromopyrimidine（RBP）based chemosensor for the selective detection of toxic elements using Hg²⁺and Cr³⁺as model elements in combination with other competitive ions such as Al³⁺,Ag⁺,Ca²⁺,Ba²⁺,Cd²⁺,Fe²⁺,Fe³⁺,Li⁺,K⁺,Mn²⁺,Mg²⁺,Cu²⁺,Na⁺,Ni²⁺,Pb²⁺and Zn²⁺.The newly developed RBP displayed good visibility,high sensitivity,excellent selectivity,high binding ability and low limit of detection in CH₃CN/HEPES buffer（1 mM 3:2 v/v,pH 7.3）as a medium.The binding constants of RBP for Hg²⁺and Cr³⁺were 1.09×10⁷ M^-1 and 8.3×10⁴ M^-1,respectively.As compared to earlier reported studies,the lowest detection limits（LOD）,i.e.,1.6mM for Hg²⁺and 4.9mM for Cr³⁺were recorded.RBP also show reversible binding affinity with Hg²⁺and Cr³⁺in the presence of ethylenediaminetetraacetate（EDTA）.The binding mode between metal ions and RBP were further investigated by using density functional theory（DFT）calculations,which support the experimental findings very well.More importantly,RBP can be prepared as a test paper kit to detect the concentration of Hg²⁺and Cr³⁺ions by changing the color of the paper visible to naked eyes and is potential for the practical infield application.In addition,the results obtained from confocal microscopy revealed that the probe is cell permeable with low cytotoxicity and can be employed as a bio-imaging reagent for intracellular recognition of Hg²⁺and Cr³⁺ratification in human breast cancer cells MCF-7.2.Self-Assembly of Alternating Copolymer vesicles for the Highly Selective,Sensitive and Visual Detection and Quantification of Aqueous Hg²⁺Self-assemblies of amphiphilic moieties into aggregates of various morphologies has been an area of great interest across chemical,physical,and biological sciences for several decades.Their extensive use in biological fields is well reported.However,for chemical sensing of toxic pollutants(Hg²⁺)there are limited reports of these aggregates.The properties of these limited reports on aqueous chemosensors for Hg²⁺are not satisfactory.Herein,in this chapter we report the vesicular chemical sensor for the aqueous detection of Hg²⁺with a high efficiency and selectivity.The vesicles were obtained through the aqueous self-assembly of the newly synthesized amphiphilic alternating copolymer poly（2,3-dihydroxybutylene-alt-2,3-dihydroxybutylenedithioether）[P（DHB-a-DHBDT）]grafted with 2-（5-bromopyridine-2-yl）-3’,6’bis（diethylamino）spiro[isoindoline-1,9’-xanthene]-3-thione（PST）probes.After the addition of Hg²⁺,the colorless aqueous solution of the vesicles changed into pink color accompanied by the significant enhancement of emission and absorption intensities at 580 nm and 559 nm,respectively.This color transition from colorless to pink allows the“bare-eye”detection of Hg²⁺ions in the aqueous medium.The vesicular sensor also shows high selectivity for Hg²⁺detection among miscellaneous metal cations.The detection limit（LOD）is about 53.0 nM,which is approximately one or two order of magnitude higher than those of the reported aqueous Hg²⁺chemosensors.The mechanism study indicates the significant enhancement of fluorescence in response to Hg²⁺should be attributed to the spirolactam ring opening via photoinduced electron transfer（PET）of xanthene moiety in the PST probes.Most importantly,the newly developed polymeric vesicle sensors also show great potential for the detection and quantification of Hg²⁺with excellent selectivity in real water samples with the percentage recovery not less than 98%.3.Vesicular chromogenic sensor for the aqueous detection and quantification of hydrazine via switch-on strategyHydrazine,a toxic environmental pollutant,which pollutes water and land,posing a sever threat to the human health.For instant valuation,a qualitative method（colorimetric）for recognition of this analyte can be a better choice.However,for precise measurement,below the threshold value,a quantitative method is needed.In this regard,the fluorometric and colorimetric approaches grasped distinctive attention of the researchers due to their sensitive,selective and biologically compatible nature.Selective detection of hydrazine is always challenging in comparison with the detection of metal ions.This limitation owes to the strong H-bonding ability and restricted modes of reaction by hydrazine with most of the solvents.In this chapter,we developed a rhodol functionalized alternating copolymer vesicle sensor for the selective,sensitive and visual detection of hydrazine.The newly engineered vesicle sensor utilizes the self-assembling behavior of P（PEGDG-a-DMSA）-g-RL followed by the strong coordination of the lone pairs present at nitrogen atom of hydrazine with electropositive carbonyl carbon atom of RL.This interaction leads to the deprotection of the fluorophore molecule attached on the surface of the polymeric vesicles.This deprotection result in the significant enhancement of spectral behavior（fluorescence and UV-Visible）of the sensor.Subsequently,this deprotection leads to the colorimetric transition of solution from colorless to pink color（visible to naked eye）.This colorimetric transition evidence the visual recognition of hydrazine for in-field mapping with ease.Furthermore,the lowest limit of detection was calculated as 3.0 nM.Finally,the aqueous quantification of hydrazine was carried out by using the distilled and raw water samples.The results obtained shows that the vesicle sensor have excellent quantification ability in terms of percentage recovery of hydrazine not less than 96%.4.Self-assembly of artificial peroxidase mimics from alternating copolymers for chromogenic detections of pollutantsEnzymes are extremely efficient at catalyzing a variety of reactions in various biological and environmental fields with high substrate specificity,activities,and yields under mild reaction conditions.However,the drawbacks of natural enzymes such as high cost,low operational stability,sensitivity to the environment etc.cannot be neglected.Therefore,the development of artificial enzymes that can overcome the disadvantages of natural enzymes are highly desired.Inspiring from the multiple functions and delicate architectures of natural peroxidases,we constructed an artificial peroxidase that can mimic as horseradish peroxidase（HRP）.This mimicking system was synthesized by the self-assembly of alternating copolymers followed by encapsulation of hemin resulted in a functional vesicle exhibiting activity like peroxidase.The thiazole moiety present in copolymers poly（ethylene glycol）diglycidyl ether-alt-bismuth-thiol P（PEGDGE-a-BTT）and 1,7-butadiene-diepoxide-alt-bismuth-thiol P（BDD-a-BTT）can easily coordinate with hemin molecule.In this complex,hemin is present as a pentacoordinate ligand having an open site for substrate binding similar to microenvironment in natural peroxidase.The as-prepared functional vesicles exhibited better stability and excellent catalytic activity in oxidizing orange II（azo dye）and phenolic compounds such as catechol and 3,3′,5,5′-tetramethylbenzidine（TMB）using hydrogen peroxide（H₂O₂）as compared to HRP over a wide pH,temperature range,and substrate concentrations.Another striking feature of these functional vesicles（alternating polymer vesicles loaded with hemin）is their use as recognition systems for chromogenic substances,indicating that these vesicles can be used as new biocatalysts for specific functions.