Optical detection of polycations via polymer film-modified microtiter plates: response mechanism and bioanalytical applications

Response Mechanism of Hyperpolarization-Based Polyion Nanosensors

The last decade has witnessed a rapid development of nano- and microparticle-based optical ion sensors, including ion-selective optodes (ISOs). While the application of nano-ISOs has shown promising performance for sensing inorganic ions, polyion sensing using nanoscale ISOs has encountered significant interference in complex samples such as blood plasma. Recently, we have reported on a new polyion sensing principle that operates through a novel mechanism to overcome this challenge. The new sensing mechanism showed improved characteristics not observed with conventional ion-exchange type sensors, but the precise mechanism of operation remained thus far unclear. This paper aims to clarify how protamine, the arginine-rich target polycation, behaves during optical signal transduction to give dramatically improved selectivity. Based on thermodynamic data, sensor performance and ζ-potential analysis, two discrete phases of protamine extraction are identified. Initially, protamine extracts into the bulk nanosensor phase, a process that is concurrent with the optical signal change. This is then followed by protamine accumulation onto the nanosensor surface, which starts only upon saturation of the optical signal change. The data indicate that the improved selectivity is due to the inability of small ions to form a sufficiently strong interaction with an active sensing ingredient, DNNS^-. Any exchange of one inorganic cation for another therefore remains optically silent, suppressing matrix effects. Moreover, the recognition of protamine is shown to be an exhaustive extraction process, making the response independent of the nature and concentration of the initial small cation in the nanosensor phase.

Protamine/heparin optical nanosensors based on solvatochromism

Optical nanosensors for the detection of polyions, including protamine and heparin, have to date relied upon ion-exchange reactions involving an analyte and an optical transducer. Unfortunately, due to the limited selectivity of the available ionophores for polyions, this mechanism has suffered from severe interference in complex sample matrices. To date no optical polyion nanosensors have demonstrated acceptable performance in serum, plasma or blood. Herein we describe a new type of nanosensor based on our discovery of a “hyper-polarizing lipophilic phase” in which dinonylnaphthalenesulfonate (DNNS⁻) polarizes a solvatochromic dye much more than even an aqueous environment. We have found that the apparent polarity of the organic phase is only modulated when DNNS⁻ binds to large polyions such as protamine, unlike singly charged ions that lack the cooperative binding required to cause a significant shift in the distribution of the polarizing DNNS⁻ ions. Our new sensing mechanism allows solvatochromic signal transduction without the transducer undergoing ion exchange. The result is significantly improved sensitivity and selectivity, enabling for the first time the quantification of protamine and heparin in human plasma using optical nanosensors that correlates with the current gold standard analysis method, the anti-Xa factor assay.

Novel optical nanosensors for the selective detection of the polycationic protamine based on solvatochromic signal change allow one to detect heparin in plasma.

Reversible pH-independent optical potassium sensor with lipophilic solvatochromic dye transducer on surface modified microporous nylon

A reversible and pH-independent fluorescent ion optode is introduced with an ionophore and surface confined solvatochromic dye transducer doped onto microporous nylon membranes.

PC-ANN assisted to the determination of Vanadium (IV) ion using an optical sensor based on immobilization of Eriochorome Cyanine R on a triacetylcellulose film

More detailed analytical studies of an optical sensor based on immobilization of Eriochorome Cyanine R (ECR) on a triacetylcellulose film have been described to determine Vanadium (IV) ions in some real samples. The sensor based on complex formation between Vanadium (IV) ions and ECR in acidic media caused the color of the film to change from violet to blue along with the appearance of a strong peak appears at 595 nm. At the optimal conditions, the calibration curve showed a linear range of 9.90×10(-7)-8.25×10(-5)mol L(-1). Vanadium (IV) ions can be detected with a detection limit of 1.03×10(-7)mol L(-1) within 15 min depending on its concentration. Also, the working range was improved by using PC-ANN algorithm. The sensor could regenerate with dilute acetic acid solution and could be completely reversible. The proposed sensor was successfully applied for determining V (IV) ions in environmental water and tea leaves.

A simple and rapid label-free fluorimetric biosensor for protamine detection based on glutathione-capped CdTe quantum dots aggregation

A novel fluorescent biosensor is developed, based on glutathione-capped CdTe quantum dots aggregation, for the determination of trace amount of an important drug, protamine. In this method with increasing the protamine concentration, the fluorescence of the quantum dots was quenched due to their aggregation. Different parameters affect the sensitivity, such as pH and the amount of the quantum dots, were optimized. Using the new optical biosensor, under the optimized conditions, protamine could be measured in the range of 2.0-200 ng mL(-1) with a detection limit of 1.0 ng mL(-)(1). The relative standard deviation for five replicates determination of 30.0 ng mL(-)(1) protamine was 1.26%. The influence of common interfering species on the protamine detection was studied. The results showed that the biosensor is highly selective and sensitive for the detection of protamine. The optical biosensor was successfully used for the determination of protamine in real samples.

Ionophore-based ion-selective optical nanosensors operating in exhaustive sensing mode

Ion selective optical sensors are typically interrogated under conditions where the sample concentration is not altered during measurement. We describe here an alternative exhaustive detection mode for ion selective optical sensors. This exhaustive sensor concept is demonstrated with ionophore-based nanooptodes either selective for calcium or the polycationic heparin antidote protamine. In agreement with a theoretical treatment presented here, linear calibration curves were obtained in the exhaustive detection mode instead of the sigmoidal curves for equilibrium-based sensors. The response range can be tuned by adjusting the nanosensor loading. The nanosensors showed average diameters of below 100 nm and the sensor response was found to be dramatically faster than that for film-based optodes. Due to the strong binding affinity of the exhaustive nanosensors, total calcium concentration in human blood plasma was successfully determined. Optical determination of protamine in human blood plasma using the exhaustive nanosensors was attempted, but was found to be less successful.

Potassium-selective optical microsensors based on surface modified polystyrene microspheres

Ion-selective microspheres based on surface modification of polystyrene particles (0.8 and 2.4 μm, diameter) are presented here for the first time.

A screening method for chiral selectors that does not require covalent attachment

A high-throughput screening protocol is proposed for chiral selector discovery. It is modeled after the protocol for biological screening of candidate drugs from chemical libraries. The procedure works based on target distribution between an aqueous phase and an organic phase. The target may be a racemate or separate enantiomers. Screening for noncovalent intermolecular association between target and candidate selectors is carried out by partitioning experiments in the presence and absence of the candidate chiral selectors in the organic phase (plasticized poly(vinyl chloride)). The partition ratio measurement uses 96-well plates for high throughput. The feasibility of this approach is validated by working with a known target/chiral selector pair, N-(3,5-dinitrobenzoyl)-alpha-phenylglycine and 2,2,2-trifluoro-1-(9-anthryl)ethanol. The validated protocol is applied to a small library of 12 cyclopropyl dipeptide isosteres. Eight bind the racemic target, econazole. Among them, one has measurable chiral selectivity. The advantage of the method is that it does not require the covalent attachment of either the analyte or the selector, and the required amount of the potential chiral selector is about 100 mug.

Fluoride-selective optical sensor based on aluminum(III)-octaethylporphyrin in thin polymeric film: further characterization and practical application

More detailed analytical studies of a new fluoride-selective optical sensor based on the use of aluminum(III)-octaethylporphyrin and a lipophilic pH indicator (4',5'-dibromofluorescein octadecyl ester; ETH-7075) within a thin plasticized poly(vinyl chloride) film are reported. The sensor exhibits extraordinary optical selectivity for fluoride over a wide range of other anions, including anions with far more positive free energies of hydration (e.g., perchlorate, thiocyanate, nitrate, etc.). UV-visible spectrophotometric studies of the sensing films indicate that fluoride interacts with the Al(III) center of the porphyrin structure, yielding both a change in the Soret band lambda(max) of the porphyrin and a change in the protonation state of the pH indicator within the film. The same change in spectral properties of the metalloporphyrin occurs in the absence of added pH indicator or with added tetraphenylborate derivative anionic sites, but optical responses to fluoride in these cases are shown to be irreversible. The presence of the pH indicator and the simultaneous fluoride/proton coextraction equilibrium chemistry is shown to greatly enhance the reversibility of fluoride binding to the Al(III) porphyrin. Optical response toward fluoride can be observed in the range of 0.1 microM-1.6 mM. Optical selectivity coefficients of <10(-6) for common anions (e.g., sulfate, chloride, nitrate, etc.) and <10(-4) for perchlorate and thiocyanate are obtained. Measurements of fluoride in drinking water via the new optical sensor are shown to correlate well with values obtained for the same samples using a classical LaF3-based fluoride ion-selective electrode method.

Electrochemical assay of protease activities based on polycation/polyanion complex as substrate and polyion sensitive membrane electrode detection

A novel electrochemical method to detect protease activities is demonstrated. The assay is based on the use of a macromolecular polycation/polyanion substrate; specifically, a complex of the arginine-rich peptide protamine and pentosan polysulfate (PPS), a highly sulfated polysaccharide. As the protease of interest cleaves the protamine within the complex into smaller fragments, free PPS is generated and detected potentiometrically via a polyanion sensitive membrane electrode. Thus, the rate of free PPS generation is proportional to the activity of the protease in the assay solution. The effect of the substrate concentration is examined, as is the influence of the protamine/PPS stoichiometry on the assay performance. Using the optimized composition and concentration of the complex, the determination of trypsin at levels down to 5 U/ml and plasmin at levels approaching 0.002 U/ml can be achieved in a 10 min period. The prospects of further adapting this scheme to determine clot-busting plasminogen activators (e.g. streptokinase, tissue plasminogen activator, etc.) in samples as complex in whole blood are discussed.