Bishnu P. Regmi

Ionic liquid-based optoelectronic sensor arrays for chemical detection

Development of ionic liquid (IL)-based colorimetric sensor arrays for detection and identification of chemicals in both the aqueous and vapor phases is reported. These facile and inexpensive optoelectronic sensors were fabricated by using ionic liquids (ILs) derived from readily available pH indicator dyes. A series of 12 different chemosensory ILs were synthesized by pairing anionic pH indicator dyes with trihexyl(tetradecyl)phosphonium ([P66614]) cation via an ion exchange reaction. The incorporation of the [P66614] cation imparted hydrophobic characteristics to these ILs, and this induced hydrophobicity led to their desired low solubility in aqueous solutions, as well as eliminated the need for a specialized hydrophobic matrix/substrate for immobilization. In this manuscript, four different matrices, i.e. glass microfiber filter papers, cotton threads, silica thin layer chromatography (TLC) plates, and alumina TLC plates, were employed for fabrication of sensor arrays. These sensor arrays were used to analyze pH values of aqueous solutions as well as for detection of acidic and basic vapors. To further prove the applicability of these IL sensor arrays as tools to sense closely related complex materials, the arrays were applied to successful discrimination of aqueous solutions of smoke from three commercially available cigarettes. The digital data generated from these sensor arrays were used in developing predictive models for accurately identifying various analytes. Two approaches were used for developing the models, and two methods were applied for assessing the predictive accuracy of the models. Use of cotton threads as a matrix led to development of a more flexible, low volume, and lightweight array to estimate pH and detect a variety of vapors. These wearable arrays may possibly be incorporated into bandages, sweatbands, diapers, and similar systems. Overall, these IL-based sensor arrays should provide a new research direction in the development of advanced colorimetric sensor arrays for detection and identification of a range of analytes relevant to many different applications.

Virtual Colorimetric Sensor Array: Single Ionic Liquid for Solvent Discrimination

There is a continuing need to develop high-performance sensors for monitoring organic solvents, primarily due to the environmental impact of such compounds. In this regard, colorimetric sensors have been a subject of intense research for such applications. Herein, we report a unique virtual colorimetric sensor array based on a single ionic liquid (IL) for accurate detection and identification of similar organic solvents and mixtures of such solvents. In this study, we employ eight alcohols and seven binary mixtures of ethanol and methanol as analytes to provide a stringent test for assessing the capabilities of this array. The UV-visible spectra of alcoholic solutions of the IL used in this study show two absorption bands. Interestingly, the ratio of absorbance for these two bands is found to be extremely sensitive to alcohol polarity. A virtual sensor array is created by using four different concentrations of IL sensor, which allowed identification of these analytes with 96.4-100% accuracy. Overall, this virtual sensor array is found to be very promising for discrimination of closely related organic solvents.

A novel composite film for detection and molecular weight determination of organic vapors

A novel vapor-sensitive composite film comprising cellulose acetate and a representative compound (1-n-butyl-2,3-dimethylimidazolium hexafluorophosphate) from a Group of Uniform Materials Based on Organic Salts (GUMBOS) has been developed and characterized. The vapor sensing characteristics of the film is investigated using a quartz crystal microbalance (QCM) transducer. The material exhibited greatly improved performance characteristics toward a number of organic vapors. It is demonstrated that the ratio of the change in resonance frequency (Δf) to the change in motional resistance (ΔR) is a concentration-independent quantity proportional to the molecular weight of the absorbed chemical species. To the best of our knowledge, this is the first study to show a direct relationship between Δf/ΔR and the molecular weight of analytes. This unique finding should prove extremely useful for easy identification and molecular weight determination of a broad range of chemical vapors.

Rational Design of QCM-D Virtual Sensor Arrays Based on Film Thickness, Viscoelasticity, and Harmonics for Vapor Discrimination

Herein, we demonstrate an alternative strategy for creating QCM-based sensor arrays by use of a single sensor to provide multiple responses per analyte. The sensor, which simulates a virtual sensor array (VSA), was developed by depositing a thin film of ionic liquid, either 1-octyl-3-methylimidazolium bromide ([OMIm][Br]) or 1-octyl-3-methylimidazolium thiocyanate ([OMIm][SCN]), onto the surface of a QCM-D transducer. The sensor was exposed to 18 different organic vapors (alcohols, hydrocarbons, chlorohydrocarbons, nitriles) belonging to the same or different homologous series. The resulting frequency shifts (Δf) were measured at multiple harmonics and evaluated using principal component analysis (PCA) and discriminant analysis (DA) which revealed that analytes can be classified with extremely high accuracy. In almost all cases, the accuracy for identification of a member of the same class, that is, intraclass discrimination, was 100% as determined by use of quadratic discriminant analysis (QDA). Impressively, some VSAs allowed classification of all 18 analytes tested with nearly 100% accuracy. Such results underscore the importance of utilizing lesser exploited properties that influence signal transduction. Overall, these results demonstrate excellent potential of the virtual sensor array strategy for detection and discrimination of vapor phase analytes utilizing the QCM. To the best of our knowledge, this is the first report on QCM VSAs, as well as an experimental sensor array, that is based primarily on viscoelasticity, film thickness, and harmonics.

Molecular weight sensing properties of ionic liquid-polymer composite films: theory and experiment

Ionic liquids (ILs) are rapidly emerging as important coating materials for highly sensitive chemical sensing devices. In this regard, we have previously demonstrated that a quartz crystal microbalance (QCM) coated with a binary mixture of an IL and cellulose acetate can be employed for detection and molecular weight estimation of organic vapors (J. Mater. Chem. 2012, 22, 13732). Herein, we report follow-up studies aimed at formulating the theoretical basis for our previously observed relationship between molecular weight and changes in the QCM parameters. In the current work, we have investigated the vapor sensing characteristics of a series of binary blends of ILs and polymers over a wider concentration range of analytes, and a quadratic equation for estimating the approximate molecular weight of an organic vapor is proposed. Additionally, the frequency (f) and dissipation factor (D) at multiple harmonics were measured by use of a quartz crystal microbalance with dissipation monitoring (QCM-D). These QCM-D data were then analyzed by fitting to various models. It is observed that the behavior of these films can be best described by use of the Maxwell viscoelastic model. In light of these observations, a plausible explanation for the correlation between the molecular weight of absorbed vapors and the QCM parameters is presented. Our previous findings appear to be a special case of this more general observation. Overall, these results underscore the true potential of IL-based composite materials for discrimination and molecular weight estimation of a broad range of chemical vapors.

Fluorescein-based ionic liquid sensor for label-free detection of serum albumins

Herein, we report a fluorescein-based room temperature ionic liquid (RTIL) as a fluorescent probe for highly selective and sensitive detection of albumins. This RTIL was prepared by pairing dianionic fluorescein (FL) with trihexyl(tetradecyl)phosphonium (P66614) cation. The ionic liquid was then dispersed in aqueous medium to produce nanodroplet dispersions. Examination of fluorescence and UV-vis spectroscopic data suggests that these dispersions are comprised of strongly fluorescent monomeric species and weakly fluorescent J-type aggregates. The relative abundance of these two types of species is observed to be dependent on the type and concentration of proteins. In the presence of bovine serum albumin (BSA) or human serum albumin (HSA), monomeric species are found to be predominant, and hence an increase in fluorescence intensity was observed with increasing concentrations of BSA or HSA. Excellent correlation between fluorescence intensity and HSA concentration was observed, and concentrations as low as 300 ng per mL of HSA were detectable. Overall, these [P66614]2[FL] nanodroplets appear to be very promising materials for facile, inexpensive, rapid, and label-free detection of albumins in aqueous medium with a high degree of accuracy, sensitivity, and selectivity.

Strategy for tuning the photophysical properties of photosensitizers for use in photodynamic therapy.

A novel approach for tuning spectral properties, as well as minimizing aggregation, in zinc porphyrin and zinc phthalocyanine-based compounds is presented. Particular emphasis is placed on use of these compounds as photosensitizers in photodynamic therapy (PDT). To accomplish this aim, a bulky hydrophobic cation, trihexyltetradecylphosphonium, is paired with anionic porphyrin and phthalocyanine dyes to produce a group of uniform materials based on organic salts (GUMBOS) that absorb at longer wavelengths with high molar absorptivity and high photostability. Nanoparticles derived from these GUMBOS possess positively charged surfaces with high zeta potential values, which are highly desirable for PDT. Upon irradiation at longer wavelengths, these GUMBOS produced singlet oxygen with greater efficiency as compared to the respective parent dyes.

GUMBOS matrices of variable hydrophobicity for matrix-assisted laser desorption/ionization mass spectrometry

RATIONALE Detection of hydrophobic peptides remains a major obstacle for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). This stems from the fact that most matrices for MALDI are hydrophilic and therefore have low affinities for hydrophobic peptides. Herein, 1-aminopyrene (AP) and AP-derived group of uniform materials based on organic salts (GUMBOS) as novel matrices for MALDI-MS analyses of peptides were investigated for hydrophobic and hydrophilic peptides. METHODS A number of solid-phase AP-based GUMBOS are synthesized with variable hydrophobicity simply by changing the counterions. Structures were confirmed by use of (1)H NMR and electrospray ionization mass spectrometry (ESI-MS). 1-Octanol/water partition coefficients (Ko/w) were used to measure the hydrophobicity of the matrices. A dried-droplet method was used for sample preparation. All spectra were obtained using a MALDI-TOF mass spectrometer in positive ion reflectron mode. RESULTS A series of AP-based GUMBOS was synthesized including [AP][chloride] ([AP][Cl]), [AP][ascorbate] ([AP][Asc]) and [AP][bis(trifluoromethane)sulfonimide] ([AP][NTf2]). The relative hydrophobicities of these compounds and α-cyano-4-hydroxycinnamic acid (CHCA, a common MALDI matrix) indicated that AP-based GUMBOS can be tuned to be much more hydrophobic than CHCA. A clear trend is observed between the signal intensities of hydrophobic peptides and hydrophobicity of the matrix. CONCLUSIONS MALDI matrices of GUMBOS with tunable hydrophobicities are easily obtained simply by varying the counterion. We have found that hydrophobic matrix materials are very effective for MALDI determination of hydrophobic peptides and, similarly, the more hydrophilic peptides displayed greater intensity in the more hydrophilic matrix.

Improving energy relay dyes for dye-sensitized solar cells by use of a group of uniform materials based on organic salts (GUMBOS)

In this study, GUMBOS (a group of uniform materials based on organic salts) derived from rhodamine B chloride, 1,1′-diethyl-2,2′-carbocyanine iodide, 3,3′-diethylthiacarbocyanine iodide, and meso-tetra(4-carboxyphenyl)porphine were synthesized and characterized for application as energy relay dyes (ERDs) in dye-sensitized solar cells (DSSCs). A facile ion exchange reaction was employed for synthesis of GUMBOS. These GUMBOS exhibited improved characteristics in comparison to their respective parent dyes, including increased solubility, thermal stability, molar extinction coefficient, and fluorescence quantum yield. In addition, excellent spectral overlap integral and Forster resonance energy transfer efficiency were observed between various GUMBOS based-ERDs (donors) and the photosensitizing dye, N719 [di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)-ruthenium(II)] (acceptor). DSSC devices were fabricated and solar efficiency was evaluated in the absence and presence of ERDs using N719 as the photosensitizing dye. DSSCs in the presence of GUMBOS-based ERDs exhibited increased solar efficiencies in comparison to DSSCs in the absence of ERDs. Moreover, increases in solar efficiencies were found to be dependent on the counterions used in GUMBOS synthesis.

Micro Gas Chromatography: An Overview of Critical Components and Their Integration

Among a number of gas analyzers, portable gas chromatography (GC) systems created by the integration of microfabricated components are promising candidates for rapid and on-site analysis of a number of complex chemical mixtures. This Feature provides a snapshot of the progress made in developing micro gas chromatography (μGC) systems in the last 4 decades. In particular, we discuss the development of microfabricated preconcentrators, separation columns, and detectors. Furthermore, we review different stationary phase materials used to coat the separation columns and the major efforts toward the development of an integrated μGC.