Organic fluorophores-based molecular probes with dual-fluorescence ratiometric responses to in-vitro/in-vivo pH for biosensing, bioimaging and biotherapeutics applications

In recent years, organic fluorophores-based molecular probes with dual-fluorescence ratiometric responses to in-vitro/in-vivo pH (DFR-MPs-pH) have been attracting much interest in fundamental application research fields. More and more scientific publications have reported the exploration of various DFR-MPs-pH systems that have unique dual-fluorescence ratiometry as the signal output, in-built and signal self-calibration functions to improve precise detection of targets. DFR-MPs-pH systems possess high-performance applications in biosensing, bioimaging and biomedicine fields. This review has comprehensively summarized recent advances of DFR-MPs-pH for the first time. First of all, the compositions and types of DFR-MPs-pH are introduced by summarizing different organic fluorophores-based molecule systems. Then, construction strategies are analyzed based on specific components, structures, properties and functions of DFR-MPs-pH. Afterward, biosensing and bioimaging applications are discussed in detail, primarily referring to pH sensing and imaging detection at the levels of living cells and small animals. Finally, biomedicine applications are fully summarized, majorly involving bio-toxicity evaluation, bio-distribution, biomedical diagnosis and therapeutics. Meanwhile, the current status, challenges and perspectives are rationally commented after detailed discussions of representative and state-of-the-art studies. Overall, this present review is comprehensive, in-time and in-depth, and can facilitate the following further exploration of new and versatile DFR-MPs-pH systems toward rational design, facile preparation, superior properties, adjustable functions and highly efficient applications in promising fields.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

Advanced polymeric nanotechnology to augment therapeutic delivery and disease diagnosis

Therapeutic and diagnostic payloads are usually associated with properties that compromise their efficacy, such as poor aqueous solubility, short half-life, low bioavailability, nonspecific accumulation and diverse side effects. Nanotechnological solutions have emerged to circumvent some of these drawbacks, augmenting therapeutic and/or diagnostic outcomes. Nanotechnology has benefited from the rise in polymer science research for the development of novel nanosystems for therapeutic and diagnostic purposes. Polymers are a widely used class of biomaterials, with a considerable number of regulatory approvals for application in clinics. In addition to their versatility in production and functionalization, several synthetic and natural polymers demonstrate biocompatible properties that dictate their successful biological performance. This article highlights the physicochemical characteristics of a variety of natural and synthetic biocompatible polymers, as well as their role in the manufacture of nanotechnology-based systems, state-of-art applications in disease treatment and diagnosis, and current challenges in finding a way to clinics.

Core–shell polymeric nanoparticles comprising BODIPY and fluorescein as ultra-bright ratiometric fluorescent pH sensors

Ultra-bright pH-sensitive fluorescent nanoparticles, including BODIPY in their core and fluorescein in their shell, are synthesized. They contain more than 2500 fluorophores and exhibit a linear fluorescence response between pH 5.5 and 7.5.

Synthesis of 1,8-naphthalimide-based probes with fluorescent switch triggered by flufenamic acid

In this work, we report the synthesis of novel fluorescent molecules, based on 1,8-naphthalimide thio- and amino-derivatives, designed to display an OFF-ON and ON-OFF photoelectron transfer fluorescence switch upon interaction with a proton-donor drug. We have functionalized the new probes to allow easy formation of a covalent link to polymer matrices, for future applications as drug delivery sensors. We have investigated the fluorescent switch of the new probes using flufenamic acid (FA, pKa 3.65), a water insoluble, non-steroidal anti-inflammatory drug, as a model drug and proton source. The rapid interaction of the new probes with FA resulted in fluorescence enhancement or decrease and a large Stokes shift, all of which allowed the detection of the drug in the nanomolar range. The new 1,8-naphthalimide fluorescent dyes reported in this work are interesting tools for the detection and quantification of acidic drugs and biomolecules.

Nanoparticle-based luminescent probes for intracellular sensing and imaging of pH

Fluorescence imaging microscopy is an essential tool in biomedical research. Meanwhile, various fluorescent probes are available for the staining of cells, cell membranes, and organelles. Though, to monitor intracellular processes and dysfunctions, probes that respond to ubiquitous chemical parameters determining the cellular function such as pH, pO2 , and Ca(2+) are required. This review is focused on the progress in the design, fabrication, and application of photoluminescent nanoprobes for sensing and imaging of pH in living cells. The advantages of using nanoprobes carrying fluorescent pH indicators compared to single molecule probes are discussed as well as their limitations due to the mostly lysosomal uptake by cells. Particular attention is paid to ratiometric dual wavelength nanosensors that enable intrinsic referenced measurements. Referencing and proper calibration procedures are basic prerequisites to carry out reliable quantitative pH determinations in complex samples such as living cells. A variety of examples will be presented that highlight the diverseness of nanocarrier materials (polymers, micelles, silica, quantum dots, carbon dots, gold, photon upconversion nanocrystals, or bacteriophages), fluorescent pH indicators for the weak acidic range, and referenced sensing mechanisms, that have been applied intracellularly up to now. WIREs Nanomed Nanobiotechnol 2016, 8:378-413. doi: 10.1002/wnan.1366 For further resources related to this article, please visit the WIREs website.

A hydrogel based nanosensor with an unprecedented broad sensitivity range for pH measurements in cellular compartments

This quadruple-labelled nanosensor has a broad sensitivity range from pH 1.4 to 7.0. It covers the full physiologically relevant range where especially the low pH range of some specialized cells can now be monitored.

Macro-/micro-environment-sensitive chemosensing and biological imaging

We have summarized the research progress on fluorescent sensors responsive to environmental factors, including local viscosity, polarity, temperature, hypoxia and pH.

Mapping the pharyngeal and intestinal pH of Caenorhabditis elegans and real-time luminal pH oscillations using extended dynamic range pH-sensitive nanosensors

Extended dynamic range pH-sensitive ratiometric nanosensors, capable of accurately mapping the full physiological pH range, have been developed and used to characterize the pH of the pharyngeal and intestinal lumen of Caenorhabditis elegans in real-time. Nanosensors, 40 nm in diameter, were prepared by conjugating pH-sensitive fluorophores, carboxyfluorescein (FAM) and Oregon Green (OG) in a 1:1 ratio, and a reference fluorophore, 5-(and-6)-carboxytetramethylrhodamine (TAMRA) to an inert polyacrylamide matrix. Accurate ratiometric pH measurements were calculated through determination of the fluorescence ratio between the pH-sensitive and reference fluorophores. Nanosensors were calibrated with an automated image analysis system and validated to demonstrate a pH measurement resolution of ±0.17 pH units. The motility of C. elegans populations, as an indicator for viability, showed nematodes treated with nanosensors, for concentrations ranging from 50.00 to 3.13 mg/mL, were not statistically different to nematodes not challenged with nanosensors up to a period of 4 days (p < 0.05). The nanosensors were also found to remain in the C. elegans lumen >24 h after nanosensor challenge was removed. The pH of viable C. elegans lumen was found to range from 5.96 ± 0.31 in the anterior pharynx to 3.59 ± 0.09 in the posterior intestine. The pharyngeal pumping rate, which dictates the transfer of ingested material from the pharynx to the intestine, was found to be temperature dependent. Imaging C. elegans at 4 °C reduced the pharyngeal pumping rate to 7 contractions/min and enabled the reconstruction of rhythmic pH oscillations in the intestinal lumen in real-time with fluorescence microscopy.

Fluorescent pH-sensitive nanoparticles in an agarose matrix for imaging of bacterial growth and metabolism

Living color: fluorescent pH-sensitive nanoparticles 12 nm in diameter were prepared and incorporated into agarose gel in a Petri dish to image pH changes during bacterial growth and metabolism.

Polystyrene nanoparticles doped with a luminescent europium complex

Polystyrene nanoparticles doped with a luminescent europium complex, Eu(tta)(3)phen, are prepared by miniemulsion polymerization. The influence of the complex on the miniemulsion polymerization is investigated by the systematic variation of the initial concentration of Eu(tta)(3)phen from 2 to 7 wt% relatively to styrene. A maximum doping level of about 2% by weight in the final particles can be achieved. At higher doping levels, destabilization of the miniemulsion leads to a loss of reproducibility with respect to both the degree of conversion and the final Eu content of the particles. Doped nanoparticles of varying diameter, ranging from 19 to 94 nm, are successfully prepared. Steady-state and time-resolved luminescence measurements indicate that the luminescence properties of Eu(tta)(3)phen in the doped latexes are unchanged from those found in THF solution. Aqueous dispersions of the doped particles exhibit characteristic red emission under UV light irradiation. The luminescence intensity increases linearly with Eu(tta)(3)phen content, indicating the absence of self-quenching despite the relatively high local concentrations within the particles.

FITC and Ru(phen)32+ co-doped silica particles as visualized ratiometric pH indicator

The performance of fluorescein isothiocyanate (FITC) and tris(1, 10-phenanathroline) ruthenium ion (Ru(phen)32+) co-doped silica particles as pH indicator was evaluated. The emission intensity ratios of the pH sensitive dye (FITC) and the reference dye (Ru(phen)32+) in the particles were dependent on pH of the environment. The changes in emission intensity ratios of the two dyes under different pH could be measured under single excitation wavelength and readily visualized by naked eye under a 365-nm UV lamp. In particular, such FITC and Ru(phen)32+ co-doped silica particles were identified to show high sensitivity to pH around the pKa of FITC (6.4), making them be potential useful as visualized pH indicator for detection of intracellular pH micro-circumstance.

Evaluating nanoparticle sensor design for intracellular pH measurements

Particle-based nanosensors have over the past decade been designed for optical fluorescent-based ratiometric measurements of pH in living cells. However, quantitative and time-resolved intracellular measurements of pH in endosomes and lysosomes using particle nanosensors are challenging, and there is a need to improve measurement methodology. In the present paper, we have successfully carried out time-resolved pH measurements in endosomes and lyosomes in living cells using nanoparticle sensors and show the importance of sensor choice for successful quantification. We have studied two nanoparticle-based sensor systems that are internalized by endocytosis and elucidated important factors in nanosensor design that should be considered in future development of new sensors. From our experiments it is clear that it is highly important to use sensors that have a broad measurement range, as erroneous quantification of pH is an unfortunate result when measuring pH too close to the limit of the sensitive range of the sensors. Triple-labeled nanosensors with a pH measurement range of 3.2-7.0, which was synthesized by adding two pH-sensitive fluorophores with different pK(a) to each sensor, seem to be a solution to some of the earlier problems found when measuring pH in the endosome-lysosome pathway.

Development of ultrabright semiconducting polymer dots for ratiometric pH sensing

Semiconducting polymer-based nanoparticles (Pdots) have recently emerged as a new class of ultrabright probes for biological detection and imaging. This paper describes the development of poly(2,5-di(3',7'-dimethyloctyl)phenylene-1,4-ethynylene) (PPE) Pdots as a platform for designing Förster resonance energy transfer (FRET)-based ratiometric pH nanoprobes. We describe and compare three routes for coupling the pH-sensitive dye, fluorescein, to PPE Pdots, which is a pH-insensitive semiconducting polymer. This approach offers a rapid and robust sensor for pH determination using the ratiometric methodology where excitation at a single wavelength results in two emission peaks, one that is pH sensitive and the other one that is pH insensitive for use as an internal reference. The linear range for pH sensing of the fluorescein-coupled Pdots is between pH 5.0 and 8.0, which is suitable for most cellular studies. The pH-sensitive Pdots show excellent reversibility and stability in pH measurements. In this paper, we use them to measure the intracellular pH in HeLa cells following their uptake by endocytosis, thus demonstrating their utility for use in cellular and imaging experiments.

Aptamers embedded in polyacrylamide nanoparticles: a tool for in vivo metabolite sensing

We describe a new type of aptamer-based optical nanosensor which uses the embedding of target responsive oligonucleotides in porous polyacrylamide nanoparticles to eliminate nuclease instability. The latter is a common problem in the use of aptamer sensors in biological environments. These aptamers embedded in nanoparticles (AptaNPs) are proposed as a tool in real-time metabolite measurements in living cells. The AptaNPs comprise 30 nm polyacrylamide nanoparticles, prepared by inverse microemulsion polymerization, which contain water-soluble aptamer switch probes (ASPs) trapped in the porous matrix of the nanoparticles. The matrix acts as a molecular fence allowing rapid diffusion of small metabolites into the particles to interact with the aptamer molecules, but at the same time it retains the larger aptamer molecules inside the nanoparticles providing protection against intracellular degradation. We tested the ability of the AptaNPs to measure the adenine-nucleotide content in yeast cells. Our results successfully demonstrate the potential for monitoring any metabolite of interest in living cells by selecting specific aptamers and embedding them in nanoparticles.