pH-dependent Optical Properties Research Articles

A Highly Sensitive and Selective Nano-Fluorescent Probe for Ratiometric and Visual Detection of Oxytetracycline Benefiting from Dual Roles of Nitrogen-Doped Carbon Dots

The specific detection of oxytetracycline (OTC) residues is significant for food safety and environmental monitoring. However, rapid specific determination of OTC from various tetracyclines is still challenging due to their similar chemical structures. Here, nitrogen-doped carbon dots (NCDs) with excitation and pH-dependent optical properties and a high-fluorescence quantum yield were successfully synthesized, which were directly employed to fabricate a dual-response fluorescence probe by self-assembly with Eu3+ (NCDs/Eu3+) for the ratiometric determination of OTC. The addition of OTC into the probe greatly enhances the characteristic emission of Eu3+ due to the "antenna effect", and the incorporation of NCDs into the probe further improves the Eu3+ fluorescence by remarkably weakening the quenching effect caused by H2O molecules and efficiently shortening the distance of energy transfer from OTC to Eu3+. Meanwhile, the fluorescence of NCDs apparently decreases due to aggregation-caused quenching. The results demonstrate that a ratiometric detection of OTC (0.1-25 µM) with a detection limit of 29 nM based on the double response signals is achieved. Additionally, visual semi-quantitative assay of OTC can be realized with the naked eye under a 365 nm UV lamp according to the fluorescence color change of the as-fabricated probe. This probe exhibits acceptable specificity and anti-interference for OTC assay, holding promise for the fast detection of OTC in real water and milk samples.

Rapid Colorimetric pH-Responsive Gold Nanocomposite Hydrogels for Sensing Applications.

Surface functionalization of metallic nanoparticles (NPs) with external groups can be engineered to fabricate sensors that are responsive to various stimuli like temperature, pH, and numerous ions. Herein, we report the synthesis of gold nanoparticles (GNPs) functionalized with 3-mercaptopropionic acid (GNPs-MPA) and the doping of these nanoparticles into hydrogel materials using the breathing-in/breathing-out (BI-BO) method. MPA has a carboxyl group that becomes protonated and, thus, ionized at a pH below its pKa (4.32); hence, the GNPs-MPA solutions and gels were mostly pH-responsive in the range of 3–5. Optical properties were assessed through ultraviolet-visible (UV-Vis) spectroscopy, namely: transmission and absorption, and the parameters used to quantify the pH changes were the full width at half maximum (FWHM) and position of surface plasmon resonance (SPR). The solutions and gels gradually changed their colors from red to indigo with pH decrementation from 5 to 3, respectively. Furthermore, the solutions’ and doped gels’ highest FWHM sensitivities towards pH variations were 20 nm and 55 nm, respectively, while the SPR’s position sensitivities were 18 nm and 10 nm, respectively. Also, transmission and scanning electron microscopy showed synchronized dispersion and aggregation of NPs with pH change in both solution and gel forms. The gel exhibited excellent repeatability and reversibility properties, and its response time was instantaneous, which makes its deployment as a colorimetric pH-triggered sensor practical. To the best of our knowledge, this is the first study that has incorporated GNPs into hydrogels utilizing the BI-BO method and demonstrated the pH-dependent optical and colorimetric properties of the developed nanocomposites.

Carbon Dots from Turnip Juice: Synthesis, Characterization and Investigation of pH-Dependent Optical Properties

In this study, carbon dots (CDs) were synthesized using a simple one-pot hydrothermal method by using turnip juice as carbon source. The water-soluble nontoxic carbon dots were obtained after reaction. The structural and optical properties of as synthesized CDs were elucidated by X-ray diffraction (XRD), Raman, Fourier Transmission Infrared (FTIR), UV–Vis absorption and Photoluminescence (PL) spectroscopy. A contour plot of the excitation dependent PL spectra of the turnip juice derived CDs and PL emission spectra in 3D color map were plotted. The maximum PL emission was found at 434 nm when excited at 364. The pH dependent luminescence properties of the CDs were investigated from pH 2-12 range in phosphate buffer solution (PBS). The resulting CDs can be evaluated in a variety of application areas, from anti-counterfeiting to bioimaging.

(Invited) Carbon Dots – from Imaging to Green Energy Applications

In recent years, nanomaterials, defined as materials < 100 nm in a single dimension, have garnered significant interest for the development of novel applications in the physical and life sciences. This is especially true for luminescent nanoparticles, which have been investigated for the development of sensors, imaging/diagnostic probes, display and solar cell applications. Recently, a relatively new class of luminescent nanomaterials, namely carbon dots, has come to light. Carbon dots, sometimes known as carbongenic dots, are carbon, oxygen, nitrogen and hydrogen containing materials with the first two elements typically accounting for ~90% of their elemental composition. Moreover, they are typically water dispersible and can be prepared from an abundant number of inexpensive sources including small molecules such as citric acid, amino acids and sugars. While they are small in size (typically 1-10 nm), they can offer a high quantum yield of emission, a process that is controlled through passivation of the surface with an organic reagent. Of particular interest are their optical properties, which can be tailored via careful selection of the starting precursors and the desired synthesis route resulting in the ability to generate fluorescence from the blue to the near infrared regions of the spectrum. In addition to their versatile optical properties, these carbon dots are generally known to have low cytotoxicity and good biocompatibility. Combined with their small size and versatile optical properties, developing carbon dots as a nanoplatform can be achieved as they lend themselves for integration in a myriad of applications most notably in bio-imaging and sensing. Our work focuses on achieving a fundamental understanding of the synthesis of carbon dots in order to control their size and achieve homogenous surface chemistry and optical properties. As they are fluorescent, we have investigated their development as bimodal imaging probes and exploited their temperature and pH dependent optical properties in order develop novel intracellular temperature and pH sensors. We can also endow them with chiral properties during synthesis opening up novel avenues for chiral sensing and antimicrobial applications. Finally, in recent developments, we have exploited their physico-chemical properties in green energy applications focusing on the production of biofuels.

PH Switchable Electrochemical and Optical Properties of Polyoctylthiophene – Pyrene Composites

AbstractPolyoctylthiophene electrochemical and optical properties are dependent on the polymer oxidation state, generally requiring presence of doping ions. In this work polyoctylthiophene composites with pyrene or aminopyrene have been prepared and studied using optical and electrochemical methods. It is shown that in the composite formed pyrene molecules act as molecular dopant for the conducting polymer, resulting in a system of partially oxidized polythiophene backbone, yet of high lipophilicity. On the other hand, composite of aminopyrene with polyoctylthiophene is characterized with pH dependent electrochemical and optical properties. The difference arises mostly from the protonation/ deprotonation reaction occurring for aminopyrene embedded in polyoctylothiophene. Nanoparticles of polythiophene‐aminopyrene show turn‐on emission dependence on increase of hydrogen ions concentration in solution in a wide pH range from 12 to 5. For low pH high emission activity is observed for nanoparticles, whereas for films high lipophilicity and low electrochemical activity pointing to presence of semiconducting polymer in the system for protonated aminopyrene. For high pH, when deprotonation of aminopyrene is observed, the system is characterized with low optical, yet high electrochemical activity and low lipophilicity. The electrochemical activity of polyoctylthiophene‐aminopyrene composite can be reversibly switched simply by solution pH change.

Spectroscopic study of the pH dependence of the optical properties of a water-soluble molecular photo-switch.

In this report we present a UV-Vis spectroscopic study of the pH dependent optical properties of 4,4'-azobenzene dicarboxylic acid in aqueous solution. A combination of chemical (acid-base) and light stimuli is combined to demonstrate that the system undergoes two types of optical switching cycles. At neutral pH the azobenzene undergoes photo-induced cis-trans isomerisation. Upon pH reduction the UV-Vis spectra show changes consistent with aggregation of the azobenzene photo-switch. The photo-responsive behaviour is dependent on the pH and conformation of the azobenzene. The optical properties of the system are dependent on the mode of pH reduction and the isomeric cis/trans composition of the photo-switch, showing hierarchical control of self-assembly.

Acid mediated tunability of stimulated laser emission from dye doped chiral microdroplets

In the last decade, the possibility to use liquid crystal droplets as optical micro-cavities and lasers has attracted much attention since it paves the way for many applications in the field of sensors or tunable photonics. Several techniques can be used to obtain small micro-resonators as, for example, dispersing a cholesteric liquid crystal inside an immiscible isotropic fluid to create an emulsion. Since liquid crystals are extremely sensitive to external factors as temperature or external fields, laser tuning can be easily achieved. Here, we report on the possibility to tune the laser emission from dye doped cholesteric liquid crystals microdroplets dispersed in a glycerol matrix in presence of nitric acid molecules in the emulsion. Using a fluorescent dye with pH dependent optical properties, the emitted laser wavelength can be tuned in a range of 60 nm. This effect could find applications for the development of spectroscopy based sensors.

Conformationally locked salicylideneaniline derivatives with strong ESIPT fluorescence

Three conformationally locked salicylideneaniline derivatives, 1a–1c, were synthesized and characterized by single-crystal X-ray diffraction. Compound 1a possesses a triply locked configuration, i.e., the intramolecular five-membered-ring C–H⋅⋅⋅N and six-membered-ring O–H⋅⋅⋅N hydrogen bonds and five-membered-ring C(1-6-7-8-9) cyclization, from which the excited-state intramolecular proton transfer takes place, resulting in a record high tautomer emission quantum yield of 0.28 in the solid state. Compared with salicylideneaniline, a substantial increase in the emission quantum yield is also observed for 1b and 1c. Furthermore, compound 1a shows pH-dependent optical properties and a highly reversible response to pH, which makes it good candidate for potential applications in pH sensing. Time-dependent density functional theory calculations are reported on these salicylideneaniline derivatives in order to rationalize their electronic structure and optical properties.

Polyethylenimine-Derived Fluorescent Nonconjugated Polymer Dots with Reversible Dual-Signal pH Response and Logic Gate Operation

A kind of nonconjugated polymer dot possessing autofluorescence was synthesized by hyperbranched polyethylenimine and salicylaldehyde via environmentally friendly imine cross-linking and self-assembly in the aqueous phase. Both fluorescence and ultraviolet–visible (UV–vis) absorption of the polymer dots showed a sensitive and reversible response to pH. The mechanism of pH-dependent optical properties was investigated. The existence of two proton-responsive functional groups (phenolic hydroxyl groups and amine groups) on the polymer dot surface is crucial. The pH-dependent fluorescence is ascribed to electron transfer, which is controlled by the protonation degree of amine groups, while the pH-dependent UV–vis absorption is induced by ionization of phenolic hydroxyl groups. Based on the pH-mediated optical properties, a colorimetric and fluorescent dual-signal pH sensor and multiple logic gates were developed. The fluorescent nonconjugated polymer dots with easy preparation and sensitive response to pH are...

PH-dependent optical properties of N-acetyl-L-cysteine-capped ZnSe(S) nanocrystals with intense/stable emissions

In the present study, a series of aqueous-based ZnSe(S) nanocrystals (NCs) was prepared at different solution pH ranging from 8 to 11.9, and using N-acetyl-L-cysteine (NAC) as capping agent. In addition to zinc blende structure, the X-ray diffraction studies demonstrated the quantum size regime of the ZnSe(S) NCs. To gain further insight toward the influence of the quantum confinement and pH values on optical properties of the as-prepared NCs, their UV-visible absorption and photoluminescence spectra were systematically analyzed. The absorption spectra experienced a red shift from ~340 to ~382 nm as the pH increased from 8.0 to 11.9, indicating the growth of the as-prepared ZnSe(S) NCs. The emission spectra also show the obvious red shift and the relative area of excitonic to trap emission, firstly increases from pH = 8.0 to 10.7, and then decreases by further increasing of the solution pH. The initial behavior might be due to the improved surface passivation of the trap dangling states by better deprotonation of thiol groups in NAC, whereas at pH >10.7, the faster growth rate of the ZnSe(s) NCs may lead to the formation of many defect sites. All of these phenomena were combined in the scheme which displays the effect of quantum confinement and solution pH on variation of the excitonic and trap-related emissions.

Facile synthesis of optical pH-sensitive molybdenum disulfide quantum dots.

An effective fabrication of MoS2 quantum dots (QDs) has been developed using alkali metal-intercalation and exfoliation. The obtained MoS2 QDs are monolayers with a uniform lateral size of 4.26 ± 0.96 nm, which exhibit distinct blue fluorescence with a quantum yield of 2.28%, robust dispersibility, storage stability and pH dependent optical properties.

Unusual Blue-Shifted Acid-Responsive Photoluminescence Behavior in 6-Amino-8-cyanobenzo[1,2-b]indolizines.

6-Amino-8-cyanobenzo[1, 2-b]indolizines, a new class of photoluminescent materials, exhibit reversible pH-dependent optical properties characterized by an uncommon and dramatic blue shift in fluorescence emission when protonated. Acid titration and NMR spectroscopy experiments reveal that, rather than the anticipated N-protonation, C-protonation and loss of aromaticity is responsible for the observed photophysical changes. Efficient synthesis from indole-2-carboxaldehydes makes variously substituted versions of this nucleus readily available to tune optical and pH effects.

Probing the pH Dependent Optical Properties of Aquatic, Terrestrial and Microbial Humic Substances by Sodium Borohydride Reduction

Chemically reducing humic (HA) and fulvic acids (FA) provides insight into spectroscopically identifiable structural moieties generating the optical properties of HA/FA from aquatic, microbial and terrestrial sources. Sodium borohydride reduction provides targeted reduction of carbonyl groups. The contrast between the pH induced optical changes of untreated, reduced and reoxidized HA/FA highlights differences in the quantity, and physicality of structural components generating optical properties associated with HA/FA. Because borohydride reactions alter pH, pH re-adjustment to the original pH is required. Careful titrations of selected HA/FA provided the mmole H+ g-1 HA/FA required to titrate reduced and reoxidized HA/FA from pH 2-11; and the pH dependent spectral slope (S) at low (pH 2-3), neutral (pH 7-7.5) and high (pH 11.0). Molar extinction coefficients (e) (Lmg-1cm-1) at pH 7.6 and 350 nm provide a point of consistency linking intrinsic pH dependent optical properties to the concentration of material used for titrations. Soil derived humic acids differ from aquatic humic acids in the heterogeneity of optically identifiable group as well as the overall concentration of those groups. The microbial source of FA has a limited concentration of homogeneous titratable groups when compared to aquatic FA generated terrestrially. Microbial FA exhibits pH linked optical recovery upon reoxidation when compared to aquatic FA which is consistent with the presence of quinones in microbial FA.

Role of Protonation State and Solvation on the pH Dependent Optical Properties of Bromocresol Green.

pH sensors play a key role in many industrial and diagnostic applications. Mostly their usage is based on experience, and in many cases the working mechanisms of these sensors are not known in detail, thereby hindering a systematic improvement of such sensors for specific applications. In this report, we present results from combined quantum chemical and molecular mechanics calculations of molecular structures and optical absorption properties of bromocresol green (BRG) in aqueous solution with varying pH value. In the acidic pH range, this chromophore has an intense band with absorption maximum at 444 nm and in the basic pH regime the absorption spectra show a redshift toward 613 nm. In order to identify the molecular structures responsible for this pH dependent optical behavior the closed and open forms of BRG are studied using static approaches considering in each case the three possible protonated states namely, neutral, anionic, and dianionic. For the most significant forms, i.e. the open forms of BRG, extensive modeling based on the integrated approach has been carried out, where the structure and dynamics were studied using hybrid QM/MM molecular dynamics, while the excitation energy calculations were carried out using time dependent density functional theory wherein the surrounding solvent was described as polarizable continuum, semicontinuum, or via a molecular mechanics force-field. The anionic and dianionic forms of BRG have been recognized as molecular forms responsible for its acidic and basic pH behavior, respectively. In contrast to the case of solvatochromic probes, the different protonation states determine the optical behavior in different pH values for pH probes. Hence, the level of solvent description appears to be of minor importance. Independent of the level of theory used to describe the solvent, all models reproduce the spectral features of BRG in different pH and also the pH induced redshift in good agreement with experiment.

Studies of pH-Sensitive Optical Properties of the deGFP1 Green Fluorescent Protein Using a Unique Polarizable Force Field.

The aim of this study is to identify the responsible molecular forms for the pH dependent optical properties of the deGFP1 green fluorescent protein mutant. We have carried out static and dynamic type calculations for all four protonation states of the chromophore to unravel the contributions due to finite temperature and the flexible protein backbone on the pH dependent optical properties. In particular, we have used a combined molecular dynamics and density functional-molecular mechanics linear response approach by means of which the optical property calculations were carried out for the chromophore in the explicitly treated solvent and bioenvironment. Two different models were used to describe the environment-electronic embedding and polarizable electronic embedding-accounting for the polarization of the chromophore and the mutual polarization between the chromophore and the environment, respectively. For this purpose a polarizable force field was derived quantum mechanically for the protein environment by use of analytical response theory. While the gas-phase calculations for the chromophore predict that the induced red shift going from low to high pH is attributed to the change of molecular forms from neutral to zwitterionic, the two more advanced models that explicitly account for the protein backbone attribute the pH shift to a neutral to anionic conversion. Some ramifications of the results for the use of GFPs as pH sensors are discussed.

Noncovalent Functionalization of Carbon Nanotubes by Fluorescent Polypeptides: Supramolecular Conjugates with pH-Dependent Absorbance and Fluorescence

Fluorescent cut single-walled carbon nanotube (CSWCNT) were prepared by simply mixing CSWCNT with water soluble rhodamine 6G (Rh6G) conjugated poly(3,4-dihydroxyphenylalanine) and poly(L-tyrosine) to form highly stable product with good dispersity in buffer solution. The optical absorbance and fluorescence spectra of the resulting fluorescent CSWCNT display interesting pH-dependent optical properties, emitting strong fluorescence only in acidic environment. Considering the extracellular pH of tumor tissue is acidic, the pH-sensitive conjugates have advantages to sense tumor cells selectively, enabling it to be utilized as a biosensor for detecting cancer cells. The protocol employed to functionalize the CSWCNT with Rh6G conjugated polypeptides in aqueous solution is proven to be direct, fast and easily scalable.

Synthesis and self-assembly of triphenylene-containing conjugated macrocycles

Two imine-based shape-persistent macrocycles containing triphenylene building blocks, along with their noncyclic model compounds, have been synthesized. Their stable conformations are predicted by quantum mechanical calculations and confirmed by 2D NOESY NMR measurements. Compared to the noncyclic model compounds, the macrocycles (3 and 6) show a higher propensity for solution self-assembly due to their stronger π–π stacking interactions, and form micro-objects which exhibit an enhanced conductivity after doping. While both macrocycles have identical molecular geometries and very similar structures, their solution self-assembly leads to micro-objects with very different shapes. The two macrocycles also show different pH-dependent optical properties.

Preparation of a pH-Sensitive Polystyrene Fluorescent Microsphere based on a Cyanine Dye

A novel pH-sensitive near-IR (NIR) fluorescent dye was synthesised and utilised in the preparation of fluorescent polystyrene microspheres. Their photochemical properties, as well as the pH-dependent optical properties were investigated and showed high sensitivity to changes of pH in the range of 3.3–8.1. The fluorescence became stronger with the increase in pH. Importantly, the microspheres were able to work in aqueous media, which is highly valuable for use in biological applications.

Layer-by-Layer Assembly of a pH-Responsive and Electrochromic Thin Film

This article summarizes an experiment on thin-film fabrication with layer-by-layer assembly that is appropriate for undergraduate laboratory courses. The purpose of this experiment is to teach students about self-assembly in the context of thin films and to expose students to the concepts of functional polymeric coatings. Students dip coat microscope slides and conductive glass with films containing polyaniline (PANi), a conducting polymer, and sulfonated polystyrene. Assembly of the film is characterized by visual inspection, UV−vis spectroscopy, and conductance measurements with a hand-held multimeter. Green-colored films with measurable conductance are apparent after two PANi layers. Subsequently, the students observe the pH-dependent optical properties of the films and also determine the electrochromic performance of the films using spectroelectrochemistry. The feasibility of this experiment has been tested by inclusion into an undergraduate-level polymer science laboratory with third- and fourth-year...

The optical properties of the blends of CdSe nanocrystals and poly( N-vinylcarbazole)

The CdSe nanocrystals with different sizes were synthesized in aqueous solution through water–sol method using l-cysteine hydrochloride as the stabilizer. The pH-dependent optical properties of the CdSe nanocrystals were investigated. Furthermore, the CdSe nanocrystals were dispersed into chloroform by using a cationic surfactant, and mixed with poly( N-vinylcarbazole) (PVK) in different mass ratios. The investigation of the photoluminescence (PL) and absorption spectra of the CdSe:PVK blends suggested that energy transfer from the PVK excited states to the CdSe nanocrystals, and it is more efficient for the smaller size nanocrystals. In the meantime, it was found that the relative emission intensity of the CdSe nanocrystals to PVK in the blends depended on the mass ratios, and the emission from the CdSe nanocrystals was the strongest as the mass ratio of CdSe to PVK was 2:1.