Paper-Based Multiplexed Colorimetric Device for the Simultaneous Detection of Salivary Biomarkers

In this study, we describe a monolithic and fully integrated paper-based device for the simultaneous detection of three prognostic biomarkers in saliva. The pattern of the proposed multiplexed device is designed with a central sample deposition zone and three identical arms, each containing a pre-treatment and test zone. Its one-step fabrication is realized by CO₂ laser cutting, providing remarkable parallelization and rapidity (ca. 5 s/device). The colorimetric detection is based on the sensitive and selective target-induced reshaping of plasmonic multibranched gold nanoparticles, which exhibit a clear spectral shift (and blue-to-pink color change) in case of non-physiological concentrations of the three salivary biomarkers. A rapid and multiplexed naked-eye or smartphone-based readout of the colorimetric response is achieved within 10 min. A prototype kit for POCT testing is also reported, providing robustness and easy handling of the device.

Plasmon switching of gold nanoparticles through thermo-responsive terminal breathing of surface-grafted DNA in hydrated ionic liquids

Self-assembly performed in ionic liquids (ILs) as a unique solvent promises distinct functions and applications in sensors, therapeutics, and optoelectronic devices due to the rich interactions between nanoparticle building blocks and ILs. However, the general consideration that common nanoparticles are readily destabilized by counterions in an IL has largely prevented researchers from investigating controlled nanoparticle assembly in IL-based systems. This study explores the assembling behaviour of double-stranded (ds) DNA-functionalized gold nanoparticles (dsDNA-AuNPs) in hydrated ionic liquids. The DNA base pair stacking assembly of dsDNA-AuNPs occurs at a low IL concentration (<5%). However, a moderate ionic liquid concentration (5-40%) can de-hybridize dsDNA and leaves single-stranded (ss) DNA stabilizing the AuNPs. In concentrated ionic liquids (>40%), interestingly, the higher ionic strength leads to the assembly of DNA-AuNPs. The triphasic assembly trend is also generally observed regardless of the type of IL. By down-regulation of DNA's melting temperature with the IL, the assembly of DNA-AuNPs affords robust response to a lower temperature range, promising applications in plasmonic devices and range-tunable temperature sensors.

Colorimetric Nanoplasmonics to Spot Hyperglycemia From Saliva

Early diagnostics and point-of-care (POC) devices can save people's lives or drastically improve their quality. In particular, millions of diabetic patients worldwide benefit from POC devices for frequent self-monitoring of blood glucose. Yet, this still involves invasive sampling processes, which are quite discomforting for frequent measurements, or implantable devices dedicated to selected chronic patients, thus precluding large-scale monitoring of the globally increasing diabetic disorders. Here, we report a non-invasive colorimetric sensing platform to identify hyperglycemia from saliva. We designed plasmonic multibranched gold nanostructures, able to rapidly change their shape and color (naked-eye detection) in the presence of hyperglycemic conditions. This "reshaping approach" provides a fast visual response and high sensitivity, overcoming common detection issues related to signal (color intensity) losses and bio-matrix interferences. Notably, optimal performances of the assay were achieved in real biological samples, where the biomolecular environment was found to play a key role. Finally, we developed a dipstick prototype as a rapid home-testing kit.

DNA Base Pair Stacking Assembly of Anisotropic Nanoparticles for Biosensing and Ordered Assembly

Anisotropic gold nanoparticles have attracted great interest due to their unique physicochemical properties derived from the shape anisotropy. Manipulation of their interfacial interactions, and thereby the assembling behaviors are often requisite in their applications ranging from optical sensing and diagnosis to self-assembly. Recently, the control of interfacial force based on base pair stacking of DNA terminals have offered a new avenue to surface engineering of nanostructures. In this review, we focus on the DNA base stacking-induced assembly of anisotropic gold nanoparticles, such as nanorods and nanotriangles. The fundamental aspects of anisotropic gold nanoparticles are provided, including the mechanism of the anisotropic growth, the properties arising from the anisotropic shape, and the construction of DNA-grafted anisotropic gold nanoparticles. Then, the advanced applications of their functional assemblies in biosensing and ordered assembly are summarized, followed by a comparison with gold nanospheres. Finally, conclusions and the direction of outlooks are given including future challenges and opportunities in this field.

Fast Adhesion of Gold Nanoparticles (AuNPs) to a Surface Using Starch Hydrogels for Characterization of Biomolecules in Biosensor Applications

Gold nanoparticles (AuNPs) are the most thoroughly studied nanoparticles because of their remarkable optical properties. Color changes in assays that use AuNPs can be easily observed with the naked eye, resulting in sensitive colorimetric methods, useful for detecting a variety of biological molecules. However, while AuNPs represent an excellent nano-platform for developing analytical methods for biosensing, there are still challenges that must be overcome before colloidal AuNPs formulation can be successfully translated into practical applications. One of those challenges is the ability to immobilize AuNPs in a solid support. There are many difficulties with controlling both the cluster size and the adhesion of the coatings formed. In addition, many of the techniques employed are expensive and time-consuming, or require special equipment. Thus, a simple and inexpensive method that only requires common lab equipment for immobilizing AuNPs on a surface using Starch Hydrogels has been developed. Starch hydrogels confer a 400% increase in stability to the nanoparticles when exposed to changes in the environment while also allowing for macromolecules to interact with the AuNPs surface. Several starch derivatives were tested, including, dextrin, beta-cyclodextrin and maltodextrin, being dextrin the one that conferred the highest stability. As a proof-of-concept, a SlipChip microfluidic sensor scheme was developed to measure the concentration of DNA in a sample. The detection limit of our biosensor was found to be 25 ng/mL and 75 ng/mL for instrument and naked eye detection, respectively.

An amplification-free colorimetric test for sensitive DNA detection based on the capturing of gold nanoparticle clusters

PCR-free or amplification-free strategies for DNA detection provide an interesting alternative to classical molecular biology techniques, opening new possibilities for on-site diagnostics. In this framework, we present herein an amplification-free colorimetric test for DNA detection, based on the capture of multiple gold nanoparticle (AuNP) clusters onto the surface of magnetic microbeads, leading to an increase of the plasmonic signal and, thus, of the overall sensitivity. Noteworthy, the assay allows the detection of as low as 15 attomoles of target DNA by simple visual inspection. The AuNP-cluster capturing mechanism was investigated by UV-vis, SEM, TEM, and EDX analysis. In a case study of E. coli contamination, the colorimetric test achieves a performance comparable to the reference instrumental PCR technique, enabling the naked-eye detection of 7.5 × 10² CFU μL^-1.

Development of Label-Free Colorimetric Assay for MERS-CoV Using Gold Nanoparticles

Worldwide outbreaks of infectious diseases necessitate the development of rapid and accurate diagnostic methods. Colorimetric assays are a representative tool to simply identify the target molecules in specimens through color changes of an indicator (e.g., nanosized metallic particle, and dye molecules). The detection method is used to confirm the presence of biomarkers visually and measure absorbance of the colored compounds at a specific wavelength. In this study, we propose a colorimetric assay based on an extended form of double-stranded DNA (dsDNA) self-assembly shielded gold nanoparticles (AuNPs) under positive electrolyte (e.g., 0.1 M MgCl2) for detection of Middle East respiratory syndrome coronavirus (MERS-CoV). This platform is able to verify the existence of viral molecules through a localized surface plasmon resonance (LSPR) shift and color changes of AuNPs in the UV-vis wavelength range. We designed a pair of thiol-modified probes at either the 5' end or 3' end to organize complementary base pairs with upstream of the E protein gene (upE) and open reading frames (ORF) 1a on MERS-CoV. The dsDNA of the target and probes forms a disulfide-induced long self-assembled complex, which protects AuNPs from salt-induced aggregation and transition of optical properties. This colorimetric assay could discriminate down to 1 pmol/μL of 30 bp MERS-CoV and further be adapted for convenient on-site detection of other infectious diseases, especially in resource-limited settings.

Target-Recycling-Amplified Colorimetric Detection of Pollen Allergen Using Non-Cross-Linking Aggregation of DNA-Modified Gold Nanoparticles

Increasing prevalence of pollen allergies has raised concerns about human health. Development of a facile and precise method to detect pollen allergens would thus be of significance for environmental assessments and medical diagnoses. Here we report a sensitive colorimetric method to detect the Japanese cedar pollen allergen, Cry j 2. The method consists of two steps: a signal amplification based on the catalytic DNA hairpin self-assembly, followed by a signal transduction using the salt-induced non-cross-linking aggregation of gold nanoparticles densely modified with short DNA. The assay exhibits a detection limit of 0.2 ng/mL, which is 130-fold greater than that of the previously reported one. Moreover, the assay enables the detection of Cry j 2 spiked in soil solutions by avoiding any interference from the contaminants. The signal amplification system includes an anti-Cry j 2 DNA aptamer, which accounts for the absence of false responses to five nontarget allergen proteins. The present method could be of general applicability to various proteins by using appropriate aptamers.

Rapid and PCR-free DNA Detection by Nanoaggregation-Enhanced Chemiluminescence

The aggregation of gold nanoparticles (AuNPs) is known to induce an enhancement of localized surface plasmon resonance due to the coupling of plasmonic fields of adjacent nanoparticles. Here we show that AuNPs aggregation also causes a significant enhancement of chemiluminescence in the presence of luminophores. The phenomenon is used to introduce a rapid and sensitive DNA detection method that does not require amplification. DNA probes conjugated to AuNPs were used to detect a DNA target sequence specific to the fungus Ceratocystis fagacearum, causal agent of oak wilt. The hybridization of the DNA target with the DNA probes results in instantaneous aggregation of AuNPs into nanoballs, leading to a significant enhancement of luminol chemiluminescence. The enhancement reveals a linear correlation (R2 = 0.98) to the target DNA concentration, with a limit of detection down to 260 fM (260 × 10-15 M), two orders of magnitude higher than the performance obtained with plasmonic colorimetry and absorption spectrometry of single gold nanoparticles. Furthermore, the detection can be performed within 22 min using only a portable luminometer.

DNA Barcoding Meets Nanotechnology: Development of a Universal Colorimetric Test for Food Authentication

Food trade globalization and the growing demand for selected food varieties have led to the intensification of adulteration cases, especially in the form of species substitution and mixing with cheaper taxa. This phenomenon has huge economic impact and sometimes even public health implications. DNA barcoding represents a well-proven molecular approach to assess the authenticity of food items, although its use is hampered by analytical constraints and timeframes that are often prohibitive for the food market. To address such issues, we have introduced a new technology, named NanoTracer, that allows for rapid and naked-eye molecular traceability of any food and requires limited instrumentation and cost-effective reagents. Moreover, unlike sequencing, this method can be used to identify not only the substitution of a fine ingredient, but also its dilution with cheaper ones.

Disulfide-induced self-assembled targets: A novel strategy for the label free colorimetric detection of DNAs/RNAs via unmodified gold nanoparticles

A modified non-cross-linking gold-nanoparticles (Au-NPs) aggregation strategy has been developed for the label free colorimetric detection of DNAs/RNAs based on self-assembling target species in the presence of thiolated probes. Two complementary thiol- modified probes, each of which specifically binds at one half of the target introduced SH groups at both ends of dsDNA. Continuous disulfide bond formation at 3' and 5' terminals of targets leads to the self-assembly of dsDNAs into the sulfur- rich and flexible products with different lengths. These products have a high affinity for the surface of Au-NPs and efficiently protect the surface from salt induced aggregation. To evaluate the assay efficacy, a small part of the citrus tristeza virus (CTV) genome was targeted, leading to a detection limit of about 5 × 10-9 mol.L-1 over a linear ranged from 20 × 10-9 to 10 × 10-7 mol.L-1. This approach also exhibits good reproducibility and recovery levels in the presence of plant total RNA or human plasma total circulating RNA extracts. Self-assembled targets can be then sensitively distinguished from non-assembled or mismatched targets after gel electrophoresis. The disulfide reaction method and integrating self-assembled DNAs/RNAs targets with bare AuNPs as a sensitive indicator provide us a powerful and simple visual detection tool for a wide range of applications.

Gold Nanoparticles: Recent Advances in the Biomedical Applications

Among the multiple branches of nanotechnology applications in the area of medicine and biology, Nanoparticle technology is the fastest growing and shows significant future promise. Nanoscale structures, with size similar to many biological molecules, show different physical and chemical properties compared to either small molecules or bulk materials, find many applications in the fields of biomedical imaging and therapy. Gold nanoparticles (AuNPs) are relatively inert in biological environment, and have a number of physical properties that are suitable for several biomedical applications. For example, AuNPs have been successfully employed in inducing localized hyperthermia for the destruction of tumors or radiotherapy for cancer, photodynamic therapy, computed tomography imaging, as drug carriers to tumors, bio-labeling through single particle detection by electron microscopy and in photothermal microscopy. Recent advances in synthetic chemistry makes it possible to make gold nanoparticles with precise control over physicochemical and optical properties that are desired for specific clinical or biological applications. Because of the availability of several methods for easy modification of the surface of gold nanoparticles for attaching a ligand, drug or other targeting molecules, AuNPs are useful in a wide variety of applications. Even though gold is biologically inert and thus shows much less toxicity, the relatively low rate of clearance from circulation and tissues can lead to health problems and therefore, specific targeting of diseased cells and tissues must be achieved before AuNPs find their application for routine human use.

Digital DNA detection based on a compact optofluidic laser with ultra-low sample consumption

DNA lasers self-amplify optical signals from a DNA analyte as well as thermodynamic differences between sequences, allowing quasi-digital DNA detection. However, these systems have drawbacks, such as relatively large sample consumption and complicated dye labelling. Moreover, although the lasing signal can detect the target DNA, it is superimposed on an unintended fluorescence background, which persists for non-target DNA samples as well. From an optical point of view, it is thus not truly digital detection and requires spectral analysis to identify the target. In this work, we propose and demonstrate an optofluidic laser that has a single layer of DNA molecules as the gain material. A target DNA produces intensive laser emission comparable to existing DNA lasers, while any unnecessary fluorescence background is successfully suppressed. As a result, the target DNA can be detected with a single laser pulse, in a truly digital manner. Since the DNA molecules cover only a single layer on the surface of the laser microcavity, the DNA sample consumption is a few orders of magnitude lower than that of existing DNA lasers. Furthermore, the DNA molecules are stained by simply immersing the microcavity in the intercalating dye solution, and thus the proposed DNA laser is free of any complex dye-labelling process prior to analysis.

Nanomaterials-based biosensors for detection of microorganisms and microbial toxins

Detection of microorganisms and microbial toxins is important for health and safety. Due to their unique physical and chemical properties, nanomaterials have been extensively used to develop biosensors for rapid detection of microorganisms with microbial cells and toxins as target analytes. In this paper, the design principles of nanomaterials-based biosensors for four selected analyte categories (bacteria cells, toxins, mycotoxins, and protozoa cells), closely associated with the target analytes' properties is reviewed. Five signal transducing methods that are less equipment intensive (colorimetric, fluorimetric, surface enhanced Raman scattering, electrochemical, and magnetic relaxometry methods) is described and compared for their sensory performance (in term oflimit of detection, dynamic range, and response time) for all analyte categories. In the end, the suitability of these five sensing principles for on-site or field applications is discussed. With a comprehensive coverage of nanomaterials, design principles, sensing principles, and assessment on the sensory performance and suitability for on-site application, this review offers valuable insight and perspective for designing suitable nanomaterials-based microorganism biosensors for a given application.

Label-Free Isothermal Amplification Assay for Specific and Highly Sensitive Colorimetric miRNA Detection

We describe a new method for the detection of miRNA in biological samples. This technology is based on the isothermal nicking enzyme amplification reaction and subsequent hybridization of the amplification product with gold nanoparticles and magnetic microparticles (barcode system) to achieve naked-eye colorimetric detection. This platform was used to detect a specific miRNA (miRNA-10b) associated with breast cancer, and attomolar sensitivity was demonstrated. The assay was validated in cell culture lysates from breast cancer cells and in serum from a mouse model of breast cancer.

Naked-eye fingerprinting of single nucleotide polymorphisms on psoriasis patients

We report a low-cost test, based on gold nanoparticles, for the colorimetric (naked-eye) fingerprinting of a panel of single nucleotide polymorphisms (SNPs), relevant for the personalized therapy of psoriasis. Such pharmacogenomic tests are not routinely performed on psoriasis patients, due to the high cost of standard technologies. We demonstrated high sensitivity and specificity of our colorimetric test by validating it on a cohort of 30 patients, through a double-blind comparison with two state-of-the-art instrumental techniques, namely reverse dot blotting and sequencing, finding 100% agreement. This test offers high parallelization capabilities and can be easily generalized to other SNPs of clinical relevance, finding broad utility in diagnostics and pharmacogenomics.

Enhanced heat diffusion in nanofluid via DNA mediated aggregation

Thermal diffusivity increases with different shapes, concentration of gold nanoparticles and also with the addition of ssDNA while the addition of dsDNA found to be ineffective in causing any kind of change in the thermal diffusivity.

Gold nanoparticles as sensitive optical probes

Recent advances in Au NP based optical sensing systems for various analytes based on absorption, fluorescence and SERS are summarized.

Rapid Non-Crosslinking Aggregation of DNA-Functionalized Gold Nanorods and Nanotriangles for Colorimetric Single-Nucleotide Discrimination

Gold nanoparticles modified with DNA duplexes are rapidly and spontaneously aggregated at high ionic strength. In contrast, this aggregation is greatly suppressed when the DNA duplex has a single-base mismatch or a single-nucleotide overhang located at the outermost surface of the particle. These colloidal features emerge irrespective of the size and composition of the particle core; however, the effects of the shape remain unexplored. Using gold nanorods and nanotriangles (nanoplatelets), we show herein that both remarkable rapidity in colloidal aggregation and extreme susceptibility to DNA structural perturbations are preserved, regardless of the shape and aspect ratio of the core. It is also demonstrated that the DNA-modified gold nanorods and nanotriangles are applicable to naked-eye detection of a single-base difference in a gene model. The current study corroborates the generality of the unique colloidal properties of DNA-functionalized nanoparticles, and thus enhances the feasibility of their practical use.

Nanoparticle multivalency counterbalances the ligand affinity loss upon PEGylation

The conjugation of receptor ligands to shielded nanoparticles is a widely used strategy to precisely control nanoparticle-cell interactions. However, it is often overlooked that a ligand's affinity can be severely impaired by its attachment to the polyethylene glycol (PEG) chains that are frequently used to protect colloids from serum protein adsorption. Using the model ligand EXP3174, a small-molecule antagonist for the angiotensin II receptor type 1 (AT1R), we investigated the ligand's affinity before and after its PEGylation and when attached to PEGylated nanoparticles. The PEGylated ligand displayed a 580-fold decreased receptor affinity compared to the native ligand. Due to their multivalency, the nanoparticles regained a low nanomolar receptor affinity, which is in the range of the affinity of the native ligand. Moreover, a four orders of magnitude higher concentration of free ligand was required to displace PEGylated nanoparticles carrying EXP3174 from the receptor. On average, one nanoparticle was decorated with 11.2 ligand molecules, which led to a multivalent enhancement factor of 22.5 compared to the monovalent PEGylated ligand. The targeted nanoparticles specifically bound the AT1R and showed no interaction to receptor negative cells. Our study shows that the attachment of a small-molecule ligand to a PEG chain can severely affect its receptor affinity. Concomitantly, when the ligand is tethered to nanoparticles, the immense avidity greatly increases the ligand-receptor interaction. Based on our results, we highly recommend the affinity testing of receptor ligands before and after PEGylation to identify potent molecules for active nanoparticle targeting.

Nanotechnology-based strategies for the detection and quantification of microRNA

MicroRNAs (miRNAs) are important regulators of gene expression, and many pathological conditions, including cancer, are characterized by altered miRNA expression levels. Therefore, accurate and sensitive quantification of miRNAs may result in correct disease diagnosis establishing these small noncoding RNA transcripts as valuable biomarkers. Aiming at overcoming some limitations of conventional quantification strategies, nanotechnology is currently providing numerous significant alternatives to miRNA sensing. In this review an up-to-date account of nanotechnology-based strategies for miRNA detection and quantification is given. The topics covered are: nanoparticle-based approaches in solution, sensing based on nanostructured surfaces, combined nanoparticle/surface sensing approaches, and single-molecule approaches.

Nanomaterials: impact on cells and cell organelles

Colloidal nanoparticles designed for the interactions with cells are very small, nanoscale objects usually consisting of inorganic cores and organic shells that are dispersed in a buffer or biological medium. By tuning the material properties of the nanoparticles a number of different biological applications of nanomaterials are enabled i.e. targeting, labelling, drug delivery, use as diagnostic tools or therapy. For all biological applications of nanoparticles, it is important to understand their interactions with the surrounding biological environment in order to predict their biological impact, in particular when designing the nanoparticles for diagnostic and therapeutic purpose. Due to the high surface-to-volume ratio, the surface of nanomaterials is very reactive. When exposed to biological fluids, the proteins and biomolecules present therein tend to associate with the nanoparticles' surface. This phenomenon is defined as biomolecular corona formation. The biomolecular corona plays a key role in the interaction between nanoparticles and biological systems, impacting on how these particles interact with biological systems on a cellular and molecular level. This book chapter describes the nature of the interactions at the bio-nano interface, shows the design strategy of nanoparticles for nanomedicine, and defines the concepts of biomolecular corona and biological identity of nanoparticles. Moreover, it describes the interaction of functionalised nanomaterials with cell organelles and intracellular fate of nanoparticles and it shows therapeutic application of gold nanoparticles as dose enhancers in radiotherapy.