Fluorescence Emission Wavelength Research Articles

Near-Infrared Liposome-Capped Au-Rare Earth Bimetallic Nanoclusters for Fluorescence Imaging of Tumor Cells

Early detection of cancer can effectively improve the survival rate of cancer patients. Fluorescence imaging has the advantages of high sensitivity and rapid imaging, and is widely used in the precise imaging detection of tumors. In this study, five kinds of Au-rare earth bimetallic nanoclusters (Au/Ln NCs) were prepared by template method using five representative rare earth elements doped with gold. The morphologies, surface charges, sizes, fluorescence quantum yields and maximum fluorescence emission wavelengths of these five kinds of Au/Ln NCs were characterized and contrasted. The findings indicated that the Au/Ce nanoclusters (Au/Ce NCs) prepared by Ce doping have the longest fluorescence emission wavelength (695 nm) and higher quantum yield, which could effectively avoid the interference of autofluorescence, and was suitable for fluorescence imaging of tumor cells. In order to improve the specific accumulation of nanoclusters in tumor cells, Au/Ce NCs were coated with folic acid modified liposomes (lip-FA) to constructed a targeted fluorescent imaging probe with near-infrared response (Au/Ce@lip-FA), which was successfully used for fluorescence imaging of tumor cells. The probe has the characteristics of stable fluorescence signal, good targeting, easy internalization, and safe metabolism, and can provide high-resolution and high-brightness imaging information, which is expected to play an important role in the clinical diagnosis and surgical treatment of tumors.

Effect of Different Thawing Methods on the Physicochemical Properties and Microstructure of Frozen Instant Sea Cucumber.

To provide recommendations to users regarding which thawing method for frozen instant sea cucumbers entails lower quality losses, in this study we compared the water retention, mechanical properties, protein properties, and microstructures of frozen instant sea cucumbers post-thawing by means of different thawing approaches, including refrigerator thawing (RT), air thawing (AT), water immersion thawing (WT), and ultrasound-assisted thawing (UT). The results indicated that UT took the shortest time. RT samples exhibited the best water-holding capacity, hardness and rheological properties, followed by UT samples. The α-helix and surface hydrophobicity of WT and AT samples were significantly lower than those of the first two methods (p < 0.05). The lowest protein maximum denaturation temperature (Tmax) was obtained by means of WT. AT samples had the lowest maximum fluorescence emission wavelength (λmax). Based on these results, WT and AT were more prone to the degradation of protein thermal stability and the destruction of the protein structure. Similarly, more crimping and fractures of the samples after WT and AT were observed in the sea cucumbers’ microstructures. Overall, we observed that UT can be used to maintain the quality of frozen instant sea cucumbers in the shortest time.

Effects of rice bran phenolics on the structure of rice bran protein under different degrees of rancidity

To explore the effects of rice bran endogenous phenolics on rice bran protein under rice bran rancidity-induced oxidation, the structure of non-dephenolized and dephenolized rice bran protein with different degrees of rice bran rancidity (rice bran was stored for 0, 1, 3, 5, and 10 d) were investigated. During rice bran storage, dephenolization increased the carbonyl (from 5.32-24.59 to 6.11–26.52 μmol/g) and free sulfhydryl (from 24.41-32.33 to 32.64–45.04 μmol/g) contents of rice bran protein, and decreased disulfide bonds content (from 24.26-26.61 to 21.78–25.42 μmol/g). Meanwhile, endogenous phenolics improved rice bran protein secondary structural flexibility. Moderate oxidation (rice bran was stored for 0 and 1 d) induced by rice bran rancidity unfolded rice bran protein. The increase of maximum fluorescence emission wavelength red shift and the improvement of surface hydrophobicity induced by endogenous phenolics under moderate oxidation indicated rice bran endogenous phenolics further promoted the unfolding of rice bran protein. Excessive oxidation (rice bran was stored for 3, 5, and 10 d) caused protein aggregation. This work demonstrated that endogenous phenolics significantly affected rice bran protein structure under oxidation, especially the synergistic effect of endogenous phenolics and moderate oxidation on protein unfolding state.

An elastic organic single crystal with bending and high pressure-induced fluorochromism properties

The mechanically tunable flexible fluorochromic elastic organic single crystals are extremely rare. Fortunately, we prepared a needle-like elastic crystal (TPABA) with bending and high pressure-induced fluorochromism properties. With the reversible change of hydrostatic pressure, the crystal fluorescence underwent reversible red-shift and blue-shift (wavelength change greater than 100 nm). Interestingly, upon bending, the inner and outer arcs on the (002) and (200) surfaces of the crystal exhibited distinct fluorochromic effects by spatially resolved μ-PL measurements. Upon bending, the fluorescence emission of the inner and outer arcs on the (002) face of the crystal was red-shifted and blue-shifted by 26 nm and 4 nm, respectively. Different from (002) face, the inner arc and outer arc of the crystal (200) plane were only blue-shifted and red-shifted by 2 nm and 3 nm, respectively. Theoretical calculations indicated that, on the one hand, with the reversible bending of the crystal, the intermolecular interaction force distance in the crystal would change reversibly, and the corresponding inner and outer arc fluorescence emission wavelengths of the crystal plane would reversibly change. On the other hand, the number of π-π interactions on each face of the crystal had an important effect on the bending-induced fluorochromism of the crystal. This paper provided new idea for the preparation of the multi-faceted bending fluorochromism flexible crystals.

Pyridine salts and aliphatic chains regulating membrane-targeted ratiometric fluorescence probe for detection of SO2 in living cells

Cell membrane is the barrier between cell and environment, which is an important structural basis for maintaining cell environment stability and regulating cell normal life activities. There are important active molecules in the cell membrane, so it is of great biological and medical significance to explore the physiological functions of active molecules in the cell membrane. In this work, probe that co-regulating cell membrane targeting by pyridine salts and lipid chains was developed. In this probe, using pyridine salt as receptor and coumarin derivatives as donor constructed a ratiometric fluorescent probe MCP based on ICT for specific recognition of SO2 derivatives. MCP was selected the cyanide group as the SO32− response site, and the introduction of pyridine propyl, which regulates hydrophobicity, so as to achieve efficient targeting of cell membranes. MCP showed high sensitivity and selectivity for SO32−. Moreover, MCP showed a fast response time (30 s) and red fluorescence emission wavelength (645 nm). In addition, MCP can successfully applying to detect endogenous SO2 on the cell membranes.

Solvatochromic Cellular Stress Sensors Reveal the Compactness Heterogeneity and Dynamics of Aggregated Proteome

Deterioration of protein homeostasis (proteostasis) often induces aberrant proteome aggregation. Visualization and dissection of the stressed proteome are of particular interest given their association with numerous degenerative diseases. Recent progress in chemical cellular stress sensors allows for direct visualization of aggregated proteome. Beyond its localization and morphology, the physicochemical nature and the dynamics of the aggregated proteome have been challenging to explore. Herein, we developed a series of solvatochromic fluorene-based D-π-A probes that can selectively and noncovalently bind to a misfolded and aggregated proteome and report on their compactness heterogeneity upon cellular stresses. We achieved this goal by variation of the heterocyclic acceptors to modulate their solvatochromism and binding affinity to amorphous aggregated proteins. The optimized sensor P6 was capable of sensing the polarity differences among different aggregated proteins via its fluorescence emission wavelength. In live cells, P6 revealed the cellular compactness heterogeneity in the aggregated proteome upon cellular stresses. Given the combinative solvatochromic and noncovalent properties, our probe can reversibly monitor the dynamic changes in the aggregated proteome compactness upon stress and after stress recovery, suggesting its potential applications in search of therapeutics to counteract disease-causing proteome stresses.

Fluorescence Emission Wavelength QSPR Application with Linear Blending Method in Machine Learning Algorithms

Machine learning tools have been developed to analyze quantitative structure-activity/property relationship (QSAR/QSPR) modeling research. Better feature selection algorithms in the ensemble methods have been used to advance QSPR/QSAR modeling, helping to understand the relation between features and target variables and reducing the computational requirements. Implementing feature importance allows for a more effective and clearer view into features' relative importance and interpret the predictions. However, the main struggle of ensemble learning methods is that each model leads to different feature selections for interpretation. Therefore, it is necessary to summarize each model and its corresponding features for better performance, resulting in high prediction accuracy. In this article, we use a blending method for prediction and interpretability in terms of the experimental values of fluorescence wavelengths. The blender requires two levels. The first level uses multiple classifiers: Random Forest, ExtraTrees, Adaptive Boosting, and Gradient Boosting. The second level requires a linear blending method that summarizes information from the classifiers. Even though the ensemble learning models accurately predict properties and activities, the algorithms are often susceptible so that even small changes can drastically impact their efficiency and accuracy. Thus, the main idea to overcome the difficulty is to implement multiple times feature selections in each model to manipulate the sensitivity. Furthermore, it accurately predicts the fluorescence data set from a regression task of the Decision Tree based (DT-based) QSAR/QSPR model. This paper provides the best-optimized features when considering specific experimental chemical or biological values. Furthermore, the tables and figures representing each model's feature selections and accuracy demonstrate the result. It shows that even though the number of features for predicting the Fluorescence Emission Wavelength reduces, the accuracy of training and test sets is maintained, and the computational effectiveness is increased.

A novel fluorescence probe for the detection of water content in organic solvents and the distinction between deuterated and nondeuterated reagents.

A novel D-π-A type fluorescent probe (probe 1) was developed for water content detection in organic solvents. By analyzing the relationship between fluorescence and water content, the probe was successfully applied to determine trace water content in tetrahydrofuran, ethyl acetate, 2-butanone, acetone, dimethylformamide, and acetonitrile. High water content in THF and ethyl acetate was associated with a gradual colour change from yellowish green to earthy yellow. The red/green value had a linear relationship with the water content in THF and ethyl acetate. There was a linear relationship between the red/blue value and water content in 2-butanone and acetone. Furthermore, probe 1 could be used for human serum albumin detection. Unexpectedly, probe 1 had a different colour response in deuterated and nondeuterated solvents, and had different fluorescence intensity and fluorescence emission wavelength. Probe 1 is rare tool that can distinguish between deuterated and nondeuterated reagents.

Study on charge transport properties and photoelectric properties of two series of 2,5-di(pyridine-2-yl)thiophene derivatives

In order to adjust the charge transfer performance and photoelectric characteristics of 2,5-di(pyridine-2-yl)thiophene (DPT) infrastructure, two series of 2,5-di(2-quinolyl)thiophene (DQT) and 2,5-di(3-isoquinolyl)thiophene (DIQT) derivatives were designed by introducing benzene, naphthalene, anthracene, tetrabenzene and pentabenzene at both ends of the molecule to extend the conjugated length. The geometric structure, orbital energy, absorption spectrum, emission spectrum and recombination energy of the designed molecules were studied by quantum chemical calculation method. The results show that the decrease of LUMO energy level and the increase of HOMO energy level can reduce Egap, which leads to the red shift of absorption wavelength and fluorescence emission wavelength; The vertical ionization potential is reduced and the vertical electron affinity is improved, thereby reducing the hole and electron injection barrier; The hole/electron recombination energy is reduced and the charge transfer ability of molecules is improved.

Stimulus-Responsiveness of Thermo-Sensitive Polymer Hybridized with N-Doped Carbon Quantum Dots and Its Applications in Solvent Recognition and Fe3+ Ion Detection

To fabricate N-CQDs hybrid thermo-sensitive polymer (poly-N-CQDs), N-doped carbon quantum dots (N-CQDs) with strong blue fluorescence and poly(N-isopropylacrylamide-co-acrylic acid) (poly(NIPAAm-co-AAc)) copolymer with thermo-sensitivity were synthesized, respectively. Subsequently, the coupling reaction between. the -COOH groups of poly(NIPAAm-co-AAc) and the -NH2 groups on the surface of the N-CQDs was carried out. The fluorescence spectra show that the coil-globule transition of the poly-N-CQDs coincided with intensity changes in the scattering peak at excitation wavelength with the temperature variations. The phase transition temperature and the fluorescent intensity of poly-N-CQDs can be regulated by modulating the composition and concentration of poly-N-CQDs as well as the temperature and pH of the local medium. The thermo-sensitivity and fluorescent properties of the poly-N-CQDs displayed good stability and reversibility. The fluorescence intensity and emission wavelengths of the poly-N-CQDs significantly changed in different solvents for solvent recognition. The poly-N-CQDs was employed as a fluorescent probe for Fe3+ detection ranging from 0.025 to 1 mM with a limit of detection (LOD) of 9.49 μM. The hybrid polymer materials have the potential to develop an N-CQDs-based thermo-sensitive device or sensor.

A near-infrared and lager stocks shift xanthene-indolium sensor for probing hydrazine in mitochondria

Hydrazine (N2H4), which can disrupt the function of mitochondria and damage the morphology of mitochondria, leading to serious diseases, was classified as a restricted substance in drinking water by the U.S. Environmental Protection Agency (EPA) and the World Health Organization (WHO). In this paper, PHNN, a mitochondrial targeting probe with near-infrared emission wavelength and large Stokes shift was constructed for hydrazine detection through the expansion of the π-conjugate system, heteroatom replacement and the introduction of nitrogen positive ions. Surprisingly, the fluorescence emission wavelength of PHNN was extended to 803 nm, and the Stokes shift was widened to 158 nm, respectively. PHNN can specifically recognize hydrazine with high sensitivity with the detection limit lower than the maximum limit of hydrazine in drinking water of EPA and WHO. Importantly, the rationally designed probe PHNN could be used for imaging hydrazine in the mitochondria of living cells, providing a potential tool for further exploration of the toxicity of hydrazine to mitochondria via visualize methods.

Screening for novel fluorescent nucleobase analogs (FBAs) using computational and experimental methods ‐ 2‐amino‐8‐vinylpurine (2A8VP), as a Case study

Fluorescent nucleobase analogs (FBAs) are molecular reporters that are structurally similar to naturally occurring nucleotides but that have significantly higher emission quantum yields and red‐shifted absorption and emission maxima. These reporter molecules play an important role in the study of structure and dynamics of nucleic acids and proteins by providing deeper insight into the mechanisms of various biological processes. The experimental synthesis and characterization of FBAs usually precedes their computational characterization. Here, we start from computation to screen for fluorescent emission wavelength and fluorescence intensity thus reducing the time and material needed in developing new novel fluorescent reporters. The photophysical properties of a putative fluorescent analog 2‐amino‐8‐vinylpurine (2A8VP) have been predicted using TD‐DFT theory. Based on these results, we have attempted its synthesis starting from 2‐amino‐6‐chloro‐purine nucleobase. Several blue‐emissive molecules have been obtained, one with a particularly high quantum yield. These reaction products have been characterized by NMR, LC‐MS, and HPLC to obtain their structural, absorbance, and fluorescence properties to compare them with the computed properties of the 2A8VP target molecule.

Carbon dots-modified paper-based chemiluminescence device for rapid determination of mercury (II) in cosmetics.

Here, a simple and portable paper-based analytical device (PAD) based on the inherent capability of carbon quantum dots (CQDs) to serve as a great emitter for the bis(2,4,6-trichlorophenyl)oxalate (TCPO)-hydrogen peroxide (H₂O₂) chemiluminescence (CL) reaction is introduced for the detection of harmful mercury ions (Hg2+ ). The energy is transferred from the unstable reaction intermediate (1,2-dioxetanedione) to CQDs, as acceptors, and an intensive orange-red CL emission is generated at ~600 nm, which is equal to the fluorescence emission wavelength of CQDs. The analytical applicability of this system was examined for the determination of Hg2+ . It was observed that Hg2+ could significantly quench the produced emission, which can be attributed to the formation of a stable and nonluminescent Hg2+ -CQDs complex. Accordingly, a simple and rapid PAD was established for monitoring Hg2+ , with a limit of detection of 0.04 μg ml-1 . No interfering effect on the signal was found from other examined cations, indicating the acceptable specificity of the method. The designed assay was appropriately utilized to detect Hg2+ ions in cosmetic samples with high efficiency. It was characterized by its low cost, ease of use, and was facile but accurate and high selective for the detection of Hg2+ ions. In addition, the portability of this probe makes it suitable for on-site screening purposes.

A reversible NIR fluorescent probe for monitoring of SO2 and formaldehyde in live cells and zebrafish

Sulfur dioxide (SO2) and formaldehyde (FA) are primary pollutants that are discharged by factories, and they can interact with each other to form secondary organic aerosols that harm human health. In organisms, SO2 can be produced by cysteine, which is also a component of GSH. The deficiency of GSH will affect the metabolism of FA. With that in mind, we developed a fluorescent probe CB-NIR that can be used to reversibly detect the volatility of SO2 and FA in vitro and in vivo in real time. The probe CB-NIR connects benzopyrylium and coumarin with conjugate bridge to reversibly detect SO2 and FA, which can emit longer NIR wavelengths. Accordingly, CB-NIR have many optical advantages, such as, high sensitivity, excellent selectivity, fast response in forward and reverse directions, large emission shift, good reversibility and naked-eye observation. In addition, to our knowledge, CB-NIR is the first reversible probe for SO2 and FA with fluorescence emission wavelength exceeding 700 nm. Furthermore, the probe can not only detect the volatility of SO2 and FA in organisms, but also monitor the time-dependent SO2 and FA changes in real time.

NIR-I region absorbing halogenated phenylamino zinc (II) phthalocyanines: Synthesis and photophysical properties

NIR-I region absorbing dyes with higher tissue penetration, lower auto-fluorescence, minimum photodamage to living tissues, and higher detection sensitivity are attracted great interesting in biological applications. In this paper, three novel NIR I absorbing halogenated phenylamino zinc (II) phthalocyanines have been firstly synthesized through a simple and practical method. They were obtained by condensation of corresponding halogenated phenylamino phthalonitriles. Halogenated phenylamino phthalonitriles were prepared by direct amination of halogenated boronic acids with nitro-phthalonitriles through a reductive molybdenum catalyzed method. The UV/Vis and fluorescence spectra of three halogenated phenylamino zinc phthalocyanines are within NIR-I region because of electron-donating conjugation effect of amine groups. In addition, the effect of substitution terminal halogen atoms as well as α/β substitution position on the UV/Vis and fluorescence spectra of three phthalocyanines were compared. Of three phthalocyanines, 3-BrZnPc with bromo-phenylamino substitution at α position possessed the longest Q-band absorption at 767 nm and fluorescence emission wavelength at 785 nm as well as the highest singlet oxygen quantum yield.

Dual-mode corroborative detection of miRNA-21 based on catalytic hairpin assembly reaction and oxygen reduction reaction combined strategy for the signal amplification

Here, we first combined the catalytic hairpin assembly (CHA) reaction and oxygen reduction reaction (ORR) to amplify the electrochemical signal for the determination of miRNA-21. In the presence of target microRNA-21, C-Ag+-C structure probe formed by hairpin DNA1 (H1) and Ag+ could be opened and catalytic hairpin assembly reaction could be carried out. Then the capture probe (C1) modified Fe3O4 @PDA@AuNPs could immobilize all the H1 and the typical oxygen reduction reaction AuNPs could be electrodeposited in situ in the I-T test. Besides, the released Ag+ in the solution could be also detected by fluorescence and colorimetry with the assistance of CdSe QDs. The fluorescence intensity and emission wavelength of CdSe QDs could be changed by the Ag+, which could reflect the concentration of miRNA-21. As a result, the limit of detection (LOD) was 323 aM. Besides, the constructed biosensor exhibited its good performance in the real samples demonstrating its practical application potential. This rarely dual-mode detection strategy makes the miRNA-21 detection results more accurate and reliable.

An Innovative Microwave-Assisted One-Step Green Synthetic Approach of Biowaste Derived Fluorescent Carbon-Dot Invisible Ink for Currency Anti-Counterfeiting Applications

The quest for the design and synthesis of carbon dots with anti-counterfeit properties that are derived via green, environmentally friendly and economical procedures is a continuous process. Carbon dots (C-dots) derived from biowaste are cheap to synthesize, possess good photo-stability and high synthetic yield, making them applicable in the anti-counterfeiting of currency. Herein, we report a novel eco-friendly, cheaper, and faster method for the synthesis of carbon dots with strong photoluminescence properties from monkey orange fruit (Strychnos spinosa) biowaste. The presence of the hydroxyl and carbonyl functional groups of the carbon dots were determined by the Fourier transform infrared spectroscopy (FTIR). The carbon dots showed strong blue emission fluorescence (emission wavelength of 452[Formula: see text]nm) when excited at 330[Formula: see text]nm. The morphology and size were determined by the atomic force microscopy (AFM) which indicated amorphous and spherical nanoparticles with an average size of less than 2[Formula: see text]nm. The no-crystallinity of the as-prepared carbon dots was confirmed using X-ray diffraction which showed the graphite-like structure. The carbon dots were produced and demonstrated good photo and chemical stability as well as high covert properties. The anti-counterfeiting of currency application by the synthesized carbon dots was demonstrated when the subsequent gel ink printed on the currency showed excellent chemical stability when exposed to washing with water, ethanol, and acetone. It also showed superior photostability when exposed to UV light at 365[Formula: see text]nm and daylight for an extended period of up to 6[Formula: see text]h. This work provides a facile, economical, and green approach for large scale production of carbon dots from the abundant biowaste.

Phenylenediamine-derived near infrared carbon dots: The kilogram-scale preparation, formation process, photoluminescence tuning mechanism and application as red phosphors

Kilogram-scale preparation of near infrared (NIR) carbon dots (C-dots) from o-phenylenediamine (o-PDA) with over 96% yield was reported in this study, and the estimated cost for each gram of C-dots was only $ 0.1. The formation process of the C-dots was elucidated and the structural model was also proposed in which the key intermediates were revealed. Excitingly, the optical properties of the C-dots could be reversibly tuned via a facile protonation-deprotonation process. Upon protonation, fluorescence emission wavelength of the C-dots could be red shifted for 47 nm, and deprotonation could enhance the photoluminescence quantum yield by three times. These optical alterations were attributed to stabilization of the excited electrons by the surface protons, and diagrams showing the electronic transition states of the C-dots were proposed according to the calculated band gaps. The as prepared C-dots demonstrated themselves as cheap and readily available phosphors for the assembling of red LEDs.

The Pursuit of Shortwave Infrared-Emitting Nanoparticles with Bright Fluorescence through Molecular Design and Excited-State Engineering of Molecular Aggregates.

Shortwave infrared (SWIR) fluorescence detection gradually becomes a pivotal real-time imaging modality, allowing one to elucidate biological complexity in deep tissues with subcellular resolution. The key challenge for the further growth of this imaging modality is the design of new brighter biocompatible fluorescent probes. This review summarizes the recent progress in the development of organic-based nanomaterials with an emphasis on new strategies that extend the fluorescence wavelength from the near-infrared to the SWIR spectral range and amplify the fluorescence brightness. We first introduce the most representative molecular design strategies to obtain near-infrared-SWIR wavelength fluorescence emission from small organic molecules. We then discuss how the formation of nanoparticles based on small organic molecules contributes to the improvement of fluorescence brightness and the shift of fluorescence to SWIR, with a special emphasis on the excited-state engineering of molecular probes in an aggregate state and spatial packing of the molecules in nanoparticles. We build our discussion based on a historical perspective on the photophysics of molecular aggregates. We extend this discussion to nanoparticles made of conjugated polymers and discuss how fluorescence characteristics could be improved by molecular design and chain conformation of the polymer molecules in nanoparticles. We conclude the article with future directions necessary to expand this imaging modality to wider bioimaging applications including single-particle deep tissue imaging. Issues related to the characterization of SWIR fluorophores, including fluorescence quantum yield unification, are also mentioned.

NIR-to-Vis Handheld Platforms for Detecting miRNA Level and Mutation Based on Sub-10 nm Sulfide Nanodots and HCR Amplification.

Sub-10 nm monodisperse alkaline-earth sulfide nanodots (ASNDs) with bright near-infrared (NIR)-excitation fluorescence and adjustable emission wavelength were prepared by a thermal decomposition method for the first time. The ASNDs exhibited high NIR-to-vis conversion efficiency and served as multicolor fluorescent labels in the proposed miR-224 assay. Targeted detection of the miR-224 level and single-nucleotide variation in miR-224 was carried out on a smartphone-based platform using a hybridization chain reaction (HCR) amplification strategy. In the presence of miR-224, the ASND-labeled HCR probes self-assembled on the surface of the diagnosis kits, generating strong fluorescent signals linearly proportional to miR-224 contents in the range of 10-2000 fM. Significantly, mutations in miR-224 led to the variation in the fluorescence intensity ratio in RGB channels. Simultaneously, evident changes of fluorescent brightness and color were easily visualized by the naked eye, which enabled on-site discrimination of miR-224 with different mutant loci. This work provides a novel preparation approach for ultrasmall NIR excitation sulfide nanodots and reveals the potential of the as-synthesized ASNDs in point-of-care (POC) nucleic acid testing. Further, it may provide a handheld platform for miRNA single-nucleotide polymorphism analysis.