Flow cytometric ion detection with plasticized poly(vinyl chloride) microspheres containing selective lonophores

Surface PEGylation of ionophore-based microspheres enables determination of serum sodium and potassium ion concentration under flow cytometry

We present here an ionophore-based ion-selective optode (ISO) platform to detect potassium and sodium concentrations in serum through flow cytometry. The ion-selective microsensors were based on polyethylene glycol (PEG)-modified polystyrene (PS) microspheres (PEG-PS). Ratiometric response curves were observed using peak channel fluorescence intensities for K⁺ (10^-6 M to 0.1 M) and Na⁺ (10^-4 M to 0.2 M) with sufficient selectivity for clinical diagnosis. Due to the matrix effect, proteins such as albumin and immunoglobulin caused an obvious increase in response for serum sample determination. To solve this problem, 4-arm PEG chains were covalently attached onto the surface of PS microspheres through a two-step reaction, which improved the stability and combated pollution of microspheres. As a preliminary application, potassium and sodium concentrations in human serums were successfully determined by the PEG-PS microsensors through flow cytometry.

Ion-Selective Nanosensor for Photoacoustic and Fluorescence Imaging of Potassium

Ion-selective optodes (ISOs), the optical analog of ion-selective electrodes, have played an increasingly important role in chemical and biochemical analysis. Here we extend this technique to ion-selective photoacoustic optodes (ISPAOs) that serve at the same time as fluorescence-based ISOs, and apply it specifically to potassium (K⁺). Notably, the potassium ion is one of the most abundant cations in biological systems, involved in numerous physiological and pathological processes. Furthermore, it has been recently reported that the presence of abnormal extracellular potassium concentrations in tumors suppresses the immune responses and thus suppresses immunotherapy. However, unfortunately, sensors capable of providing potassium images in vivo are still a future proposition. Here, we prepared an ion-selective potassium nanosensor (NS) aimed at in vivo photoacoustic (PA) chemical imaging of the extracellular environment, while being also capable of fluorescence based intracellular ion-selective imaging. This potassium nanosensor (K⁺ NS) modulates its optical properties (absorbance and fluorescence) according to the potassium concentration. The K⁺ NS is capable of measuring potassium, in the range of 1 mM to 100 mM, with high sensitivity and selectivity, by ISPAO based measurements. Also, a near infrared dye surface modified K⁺ NS allows fluorescence-based potassium sensing in the range of 20 mM to 1 M. The K⁺ NS serves thus as both PA and fluorescence based nanosensor, with response across the biologically relevant K⁺ concentrations, from the extracellular 5 mM typical values (through PA imaging) to the intracellular 150 mM typical values (through fluorescence imaging).

pH independent nano-optode sensors based on exhaustive ion-selective nanospheres

Bulk optode-based ion selective optical sensors work on the basis of extraction equilibria, and their response toward the analyte ion is known to dependent on the sample pH. This pH dependence has been one of the major disadvantages that have hampered the broad acceptance of bulk optodes in chemical sensing. We present here for the first time the use of exhaustive Ca(2+)-selective nanosensors that may overcome this pH dependent response. The nanosensors were characterized at different pH and the same linear calibration was obtained in the Ca(2+) concentration range from 10(-7) M to 10(-5) M.

Ionophore-based optical sensors

This review provides an overview of the key aspects of designing ionophore-based optical sensors (IBOS). Exact response functions are developed and compared with a simplified, generalized equation. We also provide a brief introduction into less established but promising working principles, namely dynamic response and exhaustive exchange. Absorbance and fluorescence are the main optical readout strategies used in the evaluation of a sensor response, but they usually require a robust referencing technique for real-world applications. Established referencing schemes using IBOS as well as those from other optical sensors are also discussed. Finally, the power of recently developed photoresponsive ion extraction/release systems is outlined and discussed in view of dynamically switchable IBOS or regenerative exhaustive exchange IBOS.

In vivo sodium concentration continuously monitored with fluorescent sensors

Sodium balance is vital to maintaining normal physiological function. Imbalances can occur in a variety of diseases, during certain surgical operations or during rigorous exercise. There is currently no method to continuously monitor sodium concentration in patients who may be susceptible to hyponatremia. Our approach was to design sodium specific fluorescent sensors capable of measuring physiological fluctuations in sodium concentration. The sensors are submicron plasticized polymer particles containing sodium recognition components that are coated with biocompatible poly(ethylene) glycol. Here, the sensors were brought up in saline and placed in the subcutaneous area of the skin of mice by simple injection. The fluorescence was monitored in real time using a whole animal imager to track changes in sodium concentrations. This technology could be used to monitor certain disease states or warn against dangerously low levels of sodium during exercise.

Absorbance characterization of microsphere-based ion-selective optodes

Ionophore-based microsphere sensors are characterized here in transmission mode. These sensors contain a lipophilic ionophore for the analyte cation, a chromoionophore for recognizing H+, and a lipophilic cation-exchanger. They function on the basis of an ion-exchange equilibration step where an increased concentration of analyte ion leads to increased level of extraction into the bulk of the microsphere, expelling protons in return and deprotonating the chromoionophore. Since the path length is variable across the microsphere, such bead-based sensors are normally characterized in fluorescence mode. In this paper, the response of the sensing microspheres is calculated from the ratio of transmitted light intensities at the absorbance peak maxima of the protonated and unprotonated forms of the chromoionophore. At a fixed position of the particle, the resulting responses are found to be independent of light scattering, incident light intensity and the shape or size of the microsphere. The responses of potassium-selective microspheres obtained by this method agree quantitatively with corresponding fluorescence-based data.

Multicolor Quantum Dot Encoding for Polymeric Particle-Based Optical Ion Sensors

Multicolor quantum dot-encoded polymeric microspheres are prepared with controllable and uniform doping levels that function as chemical sensors on the basis of bulk optode theory. TOP/TOPO-capped CdSe quantum dots and CdTe quantum dots capped with CdS (lambdaem = 610 and 700 nm, lambdaex = 510 nm) are blended with a THF solution of poly(methyl methacrylate-co-decyl methacrylate), poly(n-butylacrylate), or poly(vinyl chloride) plasticized with bis(2-ethylhexyl) sebacate without a need for ligand exchange. Polymeric microspheres are generated under mild, nonreactive conditions with a particle caster that breaks down a polymer stream containing the quantum dots into fine droplets by the vibration of a piezocrystal. The resulting microspheres exhibit uniform size and fluorescence emission intensities. Fluorescent bar codes are obtained by subsequent doping of two quantum dots with different colors and mass ratios into the microspheres. A linear relationship is found between the readout fluorescence ratio of the two types of nanocrystals and the mixing ratio. Quantum dot-encoded ion sensing optode microspheres are prepared by simultaneous doping of sodium ionophore X, chromoionophore II, a lipophilic tetraphenylborate cation exchanger, and TOPO-capped CdSe/CdS quantum dot as the fluorescent label. A net positive charge of the quantum dots is found to induce an anion-exchange effect on the sensor function, and therefore, an increased concentration of the lipophilic cation exchanger is required to achieve proper ion sensing properties. The modified quantum dot-labeled sodium sensing microspheres show satisfactory sodium response between 10(-4) and 0.1 M at pH 4.8, with excellent selectivity toward common interferences. The amount of the carried positive charges of the CdSe quantum dots is estimated as 2.8 mumol/g of quantum dots used in this study.

A multi-ion particle sensor

The first sub-micron polyacrylic sensor containing two independent ion-sensing systems is shown, that uses a single excitation wavelength and separates signals by using quantum dot donors to form FRET pairs with other fluorophores.

Elimination of dimer formation in InIIIporphyrin-based anion-selective membranes by covalent attachment of the ionophore

The spontaneous hydroxy-bridged dimer formation of metalloporphyrins in ion-selective membranes gives rise to a short sensor lifetime (typically days), triggered by solubility problems, the occurrence of a super-Nernstian response slope, and a pH cross response. This dimer formation is eliminated here by covalent attachment of the ionophore to the polymer matrix. Specifically, two different indium(III)porphyrins containing polymerizable groups, the chloride-selective chloro(3-[18-(3-acryloyloxypropyl)-7,12-bis(1-methoxyethyl)-3,8,13,17-tetramethylporphyrin-2-yl]propyl ester)indium(III) and the nitrite-selective Chloro(5-(4-acryloyloxyphenyl)-10,15,20-triphenylporphyrinato)indium(III), were synthesized and copolymerized with methyl methacrylate and decyl methacrylate. The covalent attachment of the ionophore to the polymer matrix indeed prevents the metalloporphyrin from forming dimeric species, as confirmed by UV/visible spectroscopy. The ion-selective membranes with grafted indium porphyrin showed Nernstian response slopes to chloride, nitrite, perchlorate, and thiocyanate anions, with a selectivity comparable to membranes with freely dissolved or underivatized metalloporphyrin. The membranes containing grafted ionophores showed a lifetime of at least two months, apparently since crystallization of the poorly soluble dimeric species may no longer occur. This is one of the first examples where the covalent attachment of an ionophore drastically improves on a number of important sensor characteristics.

K+-selective nanospheres: maximising response range and minimising response time

Cross-linked K(+) ion-selective copolymer nanospheres have been prepared by free-radical photo-initiated polymerization of n-butyl acrylate (nBA) with hexanedioldiacrylate (HDDA). Nanospheres (<200 nm) containing H(+)-chromoionophore (ETH 5294) and lipophilic salt (KTClPB) for H(+)-sensors, or ETH 5294, a K(+)-selective ionophore (valinomycin) and anionic sites for K(+)-sensors were compared, and the effect of varying the normalised concentrations for beta (R(T)(-)/L(T)) and gamma (C(m)(T)/L(T)) was studied. Experimental data were fitted to theoretical curves for the dynamic response range, based on the effect of changes in the concentration of these lipophilic sensing components incorporated into the spheres, and conditions identified for maximising the response range. A complex valinomycin-K(+) formation constant, log K(IL) = 13.13 +/- 2.22, was obtained in the nBA matrix, and from the calibration curves the apparent acid-dissociation equilibrium constant (pK(a) = 12.92 +/- 0.03) was extracted for the H(+)-sensing system, and the equilibrium exchange constant (pK(exch) = 6.16 +/- 0.03, at pH 7) calculated for the K(+)-sensing nanospheres. A basis for establishing optimum performance was identified, whereby response range and response time were balanced with maximum fluorescence yield. Parameters for achieving nanospheres with a response time <5 minutes, covering 2-3 orders of magnitude change in activity were identified, demanding nanospheres with radius <300 nm and beta(crit) approximately 0.6. An RSD(%) approximately 3% was obtained in a study of the reproducibility of the response of the proposed nanospheres, and selectivity was also evaluated for a K(+)-selective nanosensor using several cations as interfering agents. In most cases, the fluorescent emission spectra showed no response to the cations tested, confirming the selectivity of nanospheres to potassium ion. The nanosensors were satisfactorily applied to the determination of K(+) in samples mimicking physiological conditions.

Decyl Methacrylate-Based Microspot Optodes

Optode sensing membranes employing decyl methacrylate cross-linked with 1,6-hexanediol dimethacrylate as the polymer support were fabricated by a direct microspotting method on several surfaces. Photopolymerization was used to attach the microspots to the substrate. Using this method, diameters in the micrometer domain were obtained. Silanized glass, poly(methyl methacrylate) (PMMA), polycarbonate, and poly(dimethylsiloxane) were tested as possible substrates. Both polypropylene tips and the steel tips of drafting pens were used for spotting. It was determined that both silanized glass and PMMA gave working optodes, but the ones on PMMA did not fit the theoretical model. Diameters of 994 +/- 80 and 1279 +/- 85 microm were obtained on silanized glass and PMMA, respectively, using the polypropylene tips for spotting. Different size optodes were fabricated using 0.35- and 0.50-mm steel drafting pen tips. The 0.35-mm tips produced diameters of 895 +/- 26 and 688 +/- 54 microm on silanized glass and PMMA, respectively, and the 0.50-mm tips produced diameters of 1274 +/- 94 microm on silanized glass and 839 +/- 28 microm on PMMA. Thus, the microspot size can be controlled based on the hydrophobicity of the surface and the size of the tip used for spotting. Calibration plots of potassium optode microspots indicated that miniaturization does not alter response characteristics, such as selectivity, response time, and dynamic range, of the optodes.

Detection methods of microsphere based single-step bioaffinity and in vitro diagnostics assays

Microspheres provide a solid phase substrate for bioaffinity binding similar to the walls of traditional test tubes and the wells of microtiter plates. The coated microsphere concentrates analyte molecules in the reaction volume on its surface. When the bioaffinity binding reaction has reached an equilibrium, the local concentration of the analyte in close proximity of the microsphere is orders of magnitude higher than the concentration of the analyte in the total reaction volume. The preparation and quality control of microspheres coated with bioactive material is less costly and labour intensive when compared to test tube or microwell plate coating procedures. In addition, the cost for logistics and transportation of microsphere reagents is lower than that of coated tubes or plates. Moreover, microspheres can be easily used in miniaturised assay formats and several different detection schemes can be employed in the measurement of microsphere-based assays. Several different types of microspheres are commercially available. The microspheres can be manufactured in different sizes from many materials, such as polystyrene, acrylate, and glass. The surface of the microspheres can be activated to enable covalent binding of biomolecules. Further, the microspheres may contain internal fluorochrome or magnetic material, for identification or separation purposes. In this paper we review different assay formats for single-step measurement of bioaffinity assays employing microspheres. The term single-step is used to describe assays where all reagents and the sample are mixed, incubated and measured without separate washing steps.

[Microbeads, nanobeads and cytometry: applications to the analysis and purification of cells and biomolecules]

Nano and microspheres are important tools in cytometry. They have been used in first to optimize fluorescent signals detected by flow cytometry and to evaluate phagocytosis. Some antigens were also detected by using nanospheres covalently coupled to antibodies. Specifically dedicated microspheres are now widely used for antigenic quantitation by flow cytometry, and magnetic nano and micropheres are very usefull for cellular and molecular purifications. To date, analytical methods based on the use of microspheres are developed to detect proteins, nucleic acids, and ions. To this end, antibodies, oligonucleotides, or chelating agents are bound to microspheres characterized by different fluorescences. The applications of these multiplexed microspheres assays allow to identify and quantify simultaneously some macromolecules and ions, but they also permit to analyze enzymatic activities and to perform polymorphism analyses. With microspheres used as reactive support, molecular analyses are therefore possible by flow cytometry. Nano and microspheres are also usefull tools for calibration in confocal microscopy as well as for micromanipulations of biomolecules and of living cells. Inovative methods based on the use of nano and microspheres are expected in the fields of biology, medicine, food industry, and environmental sciences.

Plasticizer-free polymer containing a covalently immobilized Ca2+-selective ionophore for potentiometric and optical sensors

A derivative of a known Ca2+-selective ionophore, ETH 129, was synthesized to contain a polymerizable acrylic moiety (AU-1) and covalently grafted into a methyl methacrylate-co-decyl methacrylate polymer matrix. The polymer containing AU-1 was prepared via a simple one-step homogeneous polymerization method. It exhibited mechanical properties suitable for the fabrication of plasticizer-free ion-selective membrane electrodes and bulk optode films by solvent-casting and spin-coating techniques, respectively. The segmented sandwich membrane technique was utilized to assess the binding constant of free and covalently bound ionophores to calcium and to study their diffusion coefficients in the membrane phase. Diffusion was greatly diminished for the bound ionophore. This was confirmed in ion-selective electrode membranes containing no calcium ions in the inner solution, which should normally show apparent super-Nernstian response slopes in dilute calcium solutions. The response slope was Nernstian down to submicromolar concentration levels, indicating slow mass transport of calcium in the membrane. Optical-sensing films with the new copolymer matrix, unblended and blended with PVC-DOS, also confirmed that covalently bound ionophores are fully functional for maintaining selective ion extraction and binding properties of the sensing membrane.