Pentiptycene-Derived Fluorescence Turn-Off Polymer Chemosensor for Copper(II) Cation with High Selectivity and Sensitivity
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polymers Article Pentiptycene-Derived Fluorescence Turn-Off Polymer Chemosensor for Copper(II) Cation with High Selectivity and Sensitivity Anting Chen 1, Wei Wu 1, Megan E. A. Fegley 1, Sherryllene S. Pinnock 1, Jetty L. Duffy-Matzner 2, William E. Bernier 1 and Wayne E. Jones Jr. 1,* 1 Department of Chemistry, Binghamton University, State University of New York, Binghamton, NY 13902, USA; [email protected] (A.C.); [email protected] (W.W.); [email protected] (M.E.A.F.); [email protected] (S.S.P.); [email protected] (W.E.B.) 2 Department of Chemistry, Augustana University, Sioux Falls, SD 57197, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-607-777-2421 Academic Editor: Po-Chih Yang Received: 2 March 2017; Accepted: 21 March 2017; Published: 24 March 2017 Abstract: Fluorescent conjugated polymers (FCPs) have been explored for selective detection of metal cations with ultra-sensitivity in environmental and biological systems. Herein, a new FCP sensor, tmeda-PPpETE (poly[(pentiptycene ethynylene)-alt-(thienylene ethynylene)] with a N,N,N0-trimethylethylenediamino receptor), has been designed and synthesized via Sonogashira cross-coupling reaction with the goal of improving solid state polymer sensor development. The polymer was found to be emissive at lmax ~459 nm under UV radiation with a quantum yield of 0.119 at room temperature in THF solution. By incorporating diamino receptors and pentiptycene groups into the poly[(phenylene ethynylene)-(thiophene ethynylene)] (PPETE) backbone, the polymer showed an improved turn-off response towards copper(II) cation, with more than 99% quenching in fluorescence emission. It is capable of discriminating copper(II) cation from sixteen common cations, with a detection limit of 16.5 nM (1.04 ppb). Keywords: fluorescence; conjugated polymers; pentiptycene; chemical sensor; copper sensor 1. Introduction The development of chemical sensors has been a primary focus of environmental scientists for decades due to contamination of natural waters by toxic transition metal pollutants. Fluorescent conjugated polymers (FCPs) have received significant attention due to their ability to function as chemosensors in environmental and biological systems with both sensitivity and selectivity [1–5]. Based on rapid electron and energy transfer paths, these highly correlated one-dimensional systems have been found to demonstrate “million fold” sensitivity compared to monomolecular sensor analogues [6]. In most cases, the intrinsic electronic properties of the FCP backbones do not fulfill the selectivity requirements of most sensors. In response, various pendent groups known as “receptors” have been appended to the polymer side chains or at the end of the polymer chain [7]. Receptors such as crown ethers [8], amino group chelators [9], bi-pyridine and terpyridine ligands [10] and others have been evaluated based on their ability to bind metal cations selectively. Copper is a naturally occurring trace element found in the earth’s crust and surface water. However, it can also enter water streams through industrial deposits, wood preservatives and plumbing systems. In the U.S., the Environmental Protection Agency (EPA) has a standard action level for copper of 1.3 ppm (~20 µM) [11]. Copper is also one of the important trace elements Polymers 2017, 9, 118; doi:10.3390/polym9040118 www.mdpi.com/journal/polymers Polymers 2017, 9, 118 2 of 13 found in the living creatures. An adult human body contains about 80 mg of copper which mostly existPolymers in various 2017, 9, 118 proteins and which function via redox processes [12,13]. However, excess2 copper of 13 may damage the cellular components and result in chronic disease, neurological disorder, and liver various proteins and which function via redox processes [12,13]. However, excess copper may or kidney damage [14,15]. Copper has also been linked to Alzheimer’s, Parkinson’s and other damage the cellular components and result in chronic disease, neurological disorder, and liver or neurodegenerativekidney damage [14,15]. diseases Copper [13,16 ].has Currently, also been the li standardnked to Alzheimer’s, analytical method Parkinson’s for testing and other copper inneurodegenerative drinking water by diseases the EPA [13,16]. is absorption Currently, spectrometry the standard [17 analytical]. Other method methods for such testing as inductivelycopper in coupleddrinking plasma water mass by the spectroscopy EPA is absorption (ICP-MS) spectr [18]ometry and electrochemical [17]. Other methods methods such (cathodic as inductively or anodic strippingcoupled voltammetry) plasma mass spectroscopy [19,20] are also (ICP-MS) used in[18] other and electrochemical practice. Even method thoughs the(cathodic accuracy or anodic of these instrumentsstripping voltammetry) is high, and prized [19,20] as are “gold also standards”, used in other they practice. are quite Even expensive though and the often accuracy require of these sample preservationinstruments and is high, preparation, and prized as well as “gold as expert standards”, training. they In addition,are quite theseexpensive methods and cannotoften require be used insample biological preservation and clinical and practice, preparation, on-site as well tests as and expert in situtraining. studies. In addition, Thus, thethese need methods of developing cannot a simple,be used rapid, in biological and adaptable and clinical method practice, is rising on-site in sensor tests research.and in situ Fluorescent studies. Thus, chemosensors the need haveof beendeveloping extensively a simple, researched rapid, due and to theiradaptable fast andmethod simple is read-out,rising in highsensor selectivity research. and Fluorescent sensitivity, andchemosensors overall low price.have been extensively researched due to their fast and simple read-out, high selectivityPreviously, and thesensitivity, polymer and tmeda-PPETE overall low price. (Figure 1), N,N,N0-trimethylethylene-diamino receptors loadedPreviously, on the thiophene the polymer ring of tmeda-PPETE the poly[(phenylene (Figure ethynylene)-1), N,N,N′-trimethylethylene-dalt-(thiophene ethynylene)]iamino receptors (PPETE) backboneloaded wason the studied thiophene [9,21,22 ring]. This of polymerthe poly[(phenylene has been shown ethynylene)- to have aalt small-(thiophene fluorescence ethynylene)] “turn-on” response(PPETE) when backbone exposed wasto studied most metal[9,21,22]. cations This inpoly solution,mer has but been a significantshown to have quenching a small uponfluorescence addition “turn-on” response when exposed to most metal cations in solution, but a significant quenching of Cu2+. A more practical approach is to transfer the FCPs from solution to a solid-state matrix, and upon addition of Cu2+. A more practical approach is to transfer the FCPs from solution to a incorporate them into a field-based chemical sensing device [23–25]. Previous studies in the Jones solid-state matrix, and incorporate them into a field-based chemical sensing device [23–25]. Previous group and others have shown that the photophysical properties of the solid state chemosensors are studies in the Jones group and others have shown that the photophysical properties of the solid state significantly affected by intermolecular interactions among polymer chains [26]. This phenomenon chemosensors are significantly affected by intermolecular interactions among polymer chains [26]. has been shown to significantly reduce the quantum efficiency of light emitting diodes (LEDs) and This phenomenon has been shown to significantly reduce the quantum efficiency of light emitting sensordiodes devices. (LEDs) Thus, and sensor the transition devices. from Thus, solution the transition based sensorfrom solution to a solid based state sensor suffers to from a solid the state loss of sensitivity.suffers from In recent the loss years, of sensitivity. much effort In hasrecent been year puts, much into chemical effort has sensing been put improvements into chemical focused sensing on minimizingimprovements the extent focused of polymeron minimizing chaininteraction the extent toof providepolymer more chain efficient interaction energy to migrationprovide more along theefficient polymer energy chain migration [27,28]. along the polymer chain [27,28]. InIn the the attempt attempt to to decrease decrease the the effect effect ofof polymerpolymer self-quenching and and ππ-stacking,-stacking, a apentiptycene pentiptycene monomermonomer was was introduced introduced into into the the polymer polymer backbones.backbones. The rigid rigid three-dimensional three-dimensional sterically sterically hindered hindered frameworkframework of of the the polymer polymer backbones backbones isis knownknown toto prevent ππ-stacking-stacking of of polymer polymer chains chains [29]. [29 ].When When combinedcombined with with other other groups, groups, pentiptycene pentiptycene cancan also be part of of the the analyte analyte capture capture and and recognition recognition unitunit for for cations cations and and anions anions in in small small molecule molecule sensorssensors [[30,31].30,31]. In In the the interest interest of of developing developing cation cation recognitionrecognition FCPs FCPs containing containing the the pentiptycene pentiptycene unit,unit, thethe synthesissynthesis of of tmeda-PPpETE tmeda-PPpETE (poly[(pentiptycene (poly[(pentiptycene ethynylene)-ethynylene)-altalt-(thienylene-(thienylene ethynylene)] ethynylene)] withwith aa NN,,NN,,N′0-trimethylethylenediamino-trimethylethylenediamino