Optical detection in microscopic domains. 2. Inner filter effects for monitoring nonfluorescent molecules with fluorescence

Controlling the Release of Amphiphilic Liposomes from Alginate Hydrogel Particles for Antifouling Paint

As an alternative to the toxic antifouling paint that minimizes the adhesion force between organic molecules on large surfaces, a paint containing hydrogel particles encapsulating amphiphilic liposomes has been suggested. However, the release rate of liposomes, which is important for maximizing the antifouling performance, has not been adequately explored. We investigated the control of the release rate of liposomes encapsulated in alginate. Monodispersed alginate particles were generated using 3D-printed microfluidic devices, and their sizes were varied through the channel size, flow rate, and alginate concentration in the microfluidic devices ([Formula: see text]). The release rate of liposomes from the alginate particles was experimentally monitored under various conditions: alginate concentration, surrounding solution, and ambient fluid flow. The effects of chemical and mechanical stimuli on the effective diffusion coefficient (D_eff) of amphiphilic liposomes were analyzed, and accordingly, the best production conditions for antifouling alginate particles are suggested. This study provides essential physical insights and is useful for optimizing the performance of eco-friendly antifouling paint that includes alginate particles.

Optoacoustic sensing of hematocrit to improve the accuracy of hybrid fluorescence-ultrasound intravascular imaging

Hybrid intravascular fluorescence-ultrasound imaging is emerging for reading anatomical and biological information in vivo. By operating through blood, intravascular near-infrared fluorescence (NIRF) detection is affected by hemoglobin attenuation. Improved quantification has been demonstrated with methods that correct for the attenuation of the optical signal as it propagates through blood. These methods assume an attenuation coefficient for blood and measure the distance between detector and the vessel wall by observing the intravascular ultrasound images. Assumptions behind the attenuation employed in correction models may reduce the accuracy of these methods. Herein, we explore a novel approach to dynamically estimate optical absorption by using optoacoustic (photoacoustic) measurements. Adaptive correction is based on a trimodal intravascular catheter that integrates fluorescence, ultrasound and optoacoustic measurements. Using the novel catheter, we show how optoacoustic measurements can determine variations of blood absorption, leading to accurate quantification of the detected NIRF signals at different hematocrit values.

Colorimetric and Phosphorimetric Dual-Signaling Strategy Mediated by Inner Filter Effect for Highly Sensitive Assay of Organophosphorus Pesticides

We describe here a colorimetric and phosphorimetric dual-signaling strategy for sensitive assay of organophosphorus pesticides (OPPs). The principle for assay depends on the phenomenon that the phosphorescence of Mn-ZnS quantum dots (QDs) can be dramatically quenched by Au nanoparticles (AuNPs) through the inner filter effect (IFE) and the activity of acetylcholinesterase (AChE), an enzyme that catalytically hydrolyzes acetylthiocholine to thiocholine that can be inhibited by OPPs. By virtue of the variations of absorbance and phosphorescence of the analytical system, a dual-readout assay for OPPs has been proposed. The limits of detection for different OPPs including paraoxon, parathion, omethoate, and dimethyl dichlorovinyl phosphate (DDVP) are found to be 0.29, 0.59, 0.67, and 0.44 ng/L, respectively. The proposed assay was allowed to detect pesticides in real spiked samples and authentic contaminated apples with satisfactory results, suggesting its potential applications to detect pesticides in complicated samples.

Homogeneous fluorescence-based immunoassay via inner filter effect of gold nanoparticles on fluorescence of CdTe quantum dots

Homogeneous immunoassays are becoming more and more attractive for modern medical diagnosis because they are superior to heterogeneous immunoassays in sample and reagent consumption, analysis time, portability and disposability. Herein, a universal platform for homogeneous immunoassay, using human immunoglobulin (IgG) as a model analyte, has been developed. This assay relies upon the inner filter effect (IFE) of gold nanoparticles (AuNPs) on CdTe QDs fluorescence. The immunoreaction of antigen and antibody can induce the aggregation of antibody-functionalized AuNPs, and after aggregation the IFE of AuNPs on CdTe QDs fluorescence is greatly enhanced, resulting in a decrease of fluorescence intensity in the system. Based on this phenomenon, a wide dynamic range of 1-100 pg mL(-1) for determination of IgG can be obtained. The proposed method shows a detection limit of 0.3 pg mL(-1) for human IgG, which is much lower than the corresponding absorbance-based approach and compares favorably with other reported fluorescent methods. This immunoassay method is simple, rapid, cheap, and sensitive. The proposed method has been successfully applied to measuring IgG in serum samples, and the obtained results agreed well with those of the enzyme-linked immunosorbent assay (ELISA).

Continuous and quantitative delivery of molecules into individual cells with a diffusional microburet

Direct delivery of molecules into the cytosol of live cells is required in many areas of biology and clinical research. Molecules of interest include indicator dyes, biomolecules, and pharmacological agents. In this work we describe continuous delivery of molecules into single cells using a diffusional microburet, DMB. The DMB is a pulled glass micropipette with a fine tip that contains a microscopic plug made of a hydrogel such as agar or polyacrylamide. This plug prevents flow but allows diffusive delivery of the molecule of interest from the DMB body into the cytosol, driven by its concentration gradient. This leads to a scheme of sustained intracellular dosing that is highly reproducible and quantifiable yet does not require the addition of solution volume to the cell. Potential loss of biomolecules from the cytosol through the plug of the DMB can be greatly reduced by proper choice of the pore size and tortuosity of the hydrogel in the DMB tip. The intracellular concentration of fluorescent molecules during delivery can be obtained calibration free. In this work we demonstrate dosing of Lucifer Yellow CH, LY, a charged fluorescent dye, into individual a7r5 vascular smooth muscle cells with a DMB. New types of quantitative analytical experiments on single live cells that the DMB technology enables are titration of intracellular ions and ligands, binding sites, and efflux pathways such as those that are involved in drug resistance.

Epifluorescence imaging of electrochemically switchable Langmuir-Blodgett films of Nafion

A combination of electrochemistry and luminescence methods was exploited to obtain information on the electrochemical activity and homogeneity of Nafion Langmuir-Blodgett films. The redox behavior of the Ru(bpy)3(2+) probe incorporated in the Nafion film was monitored by epifluorescence microscopy. The photoluminescent images, recorded by a charge-coupled device (CCD) camera, reflect the distribution of the probe in the film, which resulted as very uniform, particularly in comparison with spin-coated films. Apparent diffusion coefficients (Dapp) determined by cyclic voltammetry for films of less than 10 layers are in the range of 1 x 10(-12) to 8 x 10(-12) cm(2) s(-1), that is, 2 orders of magnitude lower than values reported in the literature for spin-coated Nafion films. The application to the electrode of a potential able to oxidize the luminescent Ru(bpy)3(2+) to the nonluminescent Ru(bpy)3(3+) switched off the photoluminescence with a response time that for the LB films was much shorter than that for the spin-coated ones. Experimental evidence and calculations indicate that lowering of the film thickness down to the nanometric level is very effective in shortening the switching time, notwithstanding the lowering of the Dapp value in LB films.

Wavelength dependant quenching of 2,5-diphenyloxazole fluorescence by nucleotides

The quenching of 2,5-diphenyloxazole (PPO) fluorescence by nucleotides has been investigated by electronic absorption and steady state fluorescence spectra. Five purine nucleotides AMP, ADP, ATP, GMP and dGMP, one pyrimidine nucleotide UMP and one dinucleotide NAD have been employed in the present study. Electronic absorption studies indicate that there is no ground state complexation between the nucleotides and PPO. The quenching of PPO fluorescence was investigated at two different wavelengths. When excited at 304 nm, the lambda(max) of PPO, the fluorescence spectra of PPO is quenched following Stern-Volmer kinetics. The quenching ability of nucleotides are in the order NAD>AMP>ADP>GMP>dGMP>UMP. The K(SV) and k(q) values obtained indicate that AMP is a better quencher of PPO fluorescence than GMP, which is contrary to commonly observed pattern. The quenching is found to be dynamic in nature. However, when excited at 260 nm, the absorption maximum of the nucleotides, the fluorescence intensity of PPO is reduced with increase in the concentration of the nucleotide. This is attributed to the primary inner filter effect arising due to the absorption of the incident radiation by the nucleotides. Thus the inner filter effect phenomenon can be employed to assay the non-fluorescent molecules by fluorimetry.

A quantitative method for determination of Co(II) based on the inner filter effect of reagents on the Raman scattering signals of water

It is known that Raman scattering signals are one of main interference sources leading up to determination errors in spectrofluorometry, and thus the signals can be easily detected with a common spectrofluorometer. In this contribution, we propose a quantitative method based on the inner filter effect (IFE) of reagents on the Raman scattering signals of solvent by taking the complexation of divalent cobalt ion with 4-[(5-chloro-2-pyridyl)azo]-1,3-diaminobenzene (5-Cl-PADAB) as a model system. By adjusting the excitation wavelength of the spectrofluorometer, we could easily detect the Raman scattering signals of water at 424 nm where the maximum absorption of 5-Cl-PADAB reagent is located. In a solution of 5-Cl-PADAB, the Raman scattering signals of water are decreased owing to the IFE of 5-Cl-PADAB. If Co(II), which could form the binary complex of Co(II)/5-Cl-PADAB and consumes the 5-Cl-PADAB reagent, is present in such a case for a given amount of 5-Cl-PADAB solution, recovered Raman scattering signals could be observed and measured with a spectrofluorometer. It was found that the intensity of the enhanced Raman scattering signals is proportional to the Co(II) concentration over the range from 2.0 x 10(-7) mol L(-1) to 1.0 x 10(-5) mol L(-1), and the detection limit could reach 1.2 x 10(-7) mol L(-1). With that, Co(II) in samples could be detected with R.S.D. values lower than 2.6% and recoveries over the range of 97.2-104.7%.

Remote Fluorescence Imaging of Dynamic Concentration Profiles with Micrometer Resolution Using a Coherent Optical Fiber Bundle

Dynamic concentration profiles within the diffusion layer of an electrode were imaged in situ using fluorescence detection through a multichannel imaging fiber. In this work, a coherent optical fiber bundle is positioned orthogonal to the surface of an electrode and is used to report spatial and temporal micrometric changes in the fluorescence intensity of an initial fluorescent species. The fluorescence signal is directly related to the local concentration of a redox fluorescent reagent, which is electrochemically modulated by the electrode. Fluorescence images are collected through the optical fiber bundle during the oxidation of tris(2,2'-bipyridine)ruthenium(II) to ruthenium(III) at a diffusion-limited rate and allow the concentration profiles of Ru(II) reagent to be monitored in situ as a function of time. Tris(2,2'-bipyridine)ruthenium(II) is excited at 485 nm and emits fluorescence at 605 nm, whereas the Ru(III) oxidation state is not fluorescent. Our experiments emphasize the influence of two parameters on the micrometer spatial resolution: the numerical aperture of optical fibers within the bundle and the Ru(II) bulk concentration. The extent of the volume probed by each individual fiber of the bundle is discussed qualitatively in terms of a primary inner-filter effect and refractive index gradient. Experimentally measured fluorescence intensity profiles were found to be in very good agreement with concentration profiles predicted upon considering planar diffusion and thus validate the concept of this new application of imaging fibers. The originality of this remote approach is to provide a global view of the entire diffusion layer at a given time through one single image and to allow the time expansion of the diffusion layer to be followed quantitatively in real time.

Optical detection in microscopic domains. 3. Confocal analysis of fluorescent amphiphilic molecules

This series of papers demonstrates the feasibility of novel optical methodologies for microchemistry and analysis in aqueous samples of nano-, pico-, and femtoliter in volume. Not a conventional glass cuvette, but water-immiscible organic liquid, is used as the container for microscopic sample droplets in this approach. In part 1, absorption spectra of excellent quality were obtained and used for analysis from samples as small as a few tens of a micrometer in diameter. In part 2, an inert fluorescence marker as an internal standard was employed for indirectly detecting absorbing but nonfluorescent reagents in microsamples, employing inner filter effects. In this part 3, a third modality, confocal fluorescence microscopy, is added to the techniques being examined. A clearly visible emission ring emanating from an amphiphilic molecule, doxorubicin, at the sample boundary is demonstrated for the first time in "optically sliced" microdroplets. Relative intensity of this ring with respect to sample bulk can be used to study adsorption phenomena at liquid-liquid interfaces with proper calibrations for bulk and boundary. Quantitative separation of these two domains, a precondition to such calibrations, is also discussed.

A microscopic, continuous, optical monitor for interstitial electrolytes and glucose

Ions, such as hydrogen (pH), sodium, or potassium, as well as metabolites, such as glucose or lactate, diffuse easily between blood in the capillaries and the interstitial fluid (ISF) residing between cells, and tissues. This work represents a synthesis of several unique concepts to achieve accurate, continuous, in vivo monitoring of critical ions and glucose in the ISF under the human skin. Ionic levels are monitored using optode technology that translates the respective concentrations into variable colors of ionophore/dye/polymeric liquid membranes. Glucose is monitored indirectly, by coupling through immobilized glucose oxidase with pH, that is then detected using a similar color scheme. The monitor consists of a tiny plastic bar ("sliver sensor"), 100-300 microns wide and 1-15 mm long, placed just under the skin, with optical spots or stripes for each analyte as well as blanks for calibration. The colors are read and translated into concentration values by a watchlike device placed above the skin. Direct optical coupling between the in vivo sensing bar and the ex vivo detector device requires negligible power, and eliminates the need for wires or optical fibers crossing the skin. The microminiature sliver penetrates the skin easily and painlessly, so that the user could insert it him- or herself. No risk of track infection exists. We are reporting here on the first successful in vitro tests of this approach.