BIOPHYSICAL PROPERTIES OF NUCLEIC ACIDS AT SURFACES RELEVANT TO MICROARRAY PERFORMANCE

Conformationally Locked Peptide-DNA Nanostructures for CRISPR-Amplified Activity-Based Sensing

We introduce a new class of chemical probes for activity-based sensing of proteases, termed cleavable, locked initiator probes (CLIPs). CLIPs contain a protease-cleavable peptide linked between two programmable DNA strands-an "initiator" DNA and a shorter "blocking" DNA. These DNA sequences are designed to hybridize, creating a "locked" hairpin-like structure. Upon proteolytic cleavage, the initiator strand is released, triggering the activation of CRISPR-Cas12a enzymes and producing an amplified fluorescence response. CLIPs generate more than 20-fold turn-on signals at room temperature (25 °C), significantly outperforming commercial probes by yielding ∼40-fold lower limits of detection (LOD) at 100-fold lower concentrations. Their versatility enables the detection of various disease-relevant proteases-including the SARS-CoV-2 main protease, caspase-3, matrix metalloproteinase-7, and cathepsin B-simply by altering the peptide sequence. Importantly, CLIPs detect cathepsin B in four different colorectal cancer cell lines, highlighting their clinical potential. Taken together, the sensitivity (LOD: ∼88 pM), selectivity, and rapid assay time (down to 35 min), combined with the ability to operate in complex biological media with minimal sample preparation, position CLIPs as powerful chemical tools for activity-based sensing of functional enzymes.

Tailoring DNA Surface Interactions on Single-Layer Graphene: Comparative Analysis of Pyrene, Acridine, and Fluorenyl Methyl Linkers

This study investigates the effect of different linkers and solvents on the immobilization of DNA probes on graphene surfaces, which are crucial for developing high-performance biosensors. Quartz crystal microbalance with dissipation (QCM-D) measurements were used to characterize in situ and real-time the immobilization of ssDNA and hybridization efficiency on model graphene surfaces. The DNA probes immobilization kinetics and thermodynamics were systematically investigated for all the pairings between three bifunctional linkers─1-pyrenebutyric acid succinimidyl ester (PBSE), Fluorenylmethylsuccinimidyl carbonate (FSC), and Acridine Orange (AO) succinimidyl ester─and three organic solvents (DMF, DMSO, and 10% DMF/ethanol). The linker's spatial orientation and effective surface modification for DNA probe attachment were also evaluated based on footprints and DNA probe surface coverage. Graphene surfaces functionalized with PBSE in DMF achieved the highest DNA probe surface density (up to 1.31 × 1013 molecules cm-2) and fastest kinetic, p values above 4, and hybridization efficiencies of at least 70%, with 20 to 30% of ssDNA directly adsorbed nonspecifically on the functionalized graphene surface, which has significant implications for the design of sensitive biosensors. The efficiency of the ethanolamine-NHS blocking reaction was estimated to be 80%. The surface packing density of the linker was estimated at 25% of the entire surface coverage for PBSE, and about 22 and 13% for AO and FSC, respectively. Overall, the surface coverage achieved for probe DNA was in the same order of magnitude as that obtained on flat gold surfaces (≥1013 molecules cm-2), typically used in biosensors. These findings highlight the importance of the selected conditions for graphene surface modification to achieve high DNA probe surface density on graphene materials. These results underscore the critical role of interface engineering in achieving target functional outcomes in biosensing technology.

Bifunctional Protein TC1 Mediated One-Pot Strategy for Robust Immobilization of DNA with High Accessibility

Immobilizing DNA with high accessibility at the interface is attractive but challenging. Current methods often involve multiple chemical reactions and derivatives. In this study, an endonuclease, TC1, is introduced to develop a robust strategy for immobilizing DNA with enhanced accessibility. TC1 enables direct immobilization of DNA onto a solid support through self-catalytic DNA covalent coupling and robust solid adsorption capabilities. This method demonstrates high accessibility to target molecules, supported by the improved sensitivity of DNA hybridization and aptamer-target recognition assays. TC1-mediated DNA immobilization is a one-pot reaction that does not require chemical derivatives, making it promising for the development of high-performance DNA materials and technologies.

Correlated Hybrid DNA Structures Explored by the oxDNA Model

Thermodynamically, perfect DNA hybridization can be formed between probes and their corresponding targets due to the favorable energy. However, this is not the case dynamically. Here, we use molecular dynamics (MD) simulations based on the oxDNA model to investigate the process of DNA microarray hybridization. In general, correlated hybrid DNA structures are formed, including one probe associated with several targets as well as one target hybrid with multiple probes leading to the target-mediated hybridization. The formation of these two types of correlated structures largely depends on the surface coverage of the DNA microarray. Moreover, DNA sequence, DNA length, and spacer length have an impact on the structural formation. Our findings shed light on the dynamics of DNA hybridization, which is important for the application of DNA microarray.

DNA-Barcoded Plasmonic Nanostructures for Activity-Based Protease Sensing

We report the development of a new class of protease activity sensors called DNA-barcoded plasmonic nanostructures. These probes are comprised of gold nanoparticles functionalized with peptide-DNA conjugates (GPDs), where the peptide is a substrate of the protease of interest. The DNA acts as a barcode identifying the peptide and facilitates signal amplification. Protease-mediated peptide cleavage frees the DNA from the nanoparticle surface, which is subsequently measured via a CRISPR/Cas12a-based assay as a proxy for protease activity. As proof-of-concept, we show activity-based, multiplexed detection of the SARS-CoV-2-associated protease, 3CL, and the apoptosis marker, caspase 3, with high sensitivity and selectivity. GPDs yield >25-fold turn-on signals, 100-fold improved response compared to commercial probes, and detection limits as low as 58 pM at room temperature. Moreover, nanomolar concentrations of proteases can be detected visually by leveraging the aggregation-dependent color change of the gold nanoparticles. We showcase the clinical potential of GPDs by detecting a colorectal cancer-associated protease, cathepsin B, in three different patient-derived cell lines. Taken together, GPDs detect physiologically relevant concentrations of active proteases in challenging biological samples, require minimal sample processing, and offer unmatched multiplexing capabilities (mediated by DNA), making them powerful chemical tools for biosensing and disease diagnostics.

DNA Sensor for the Detection of Brucella spp. Based on Magnetic Nanoparticle Markers

Due to the limitations of conventional Brucella detection methods, including safety concerns, long incubation times, and limited specificity, the development of a rapid, selective, and accurate technique for the early detection of Brucella in livestock animals is crucial to prevent the spread of the associated disease. In the present study, we introduce a magnetic nanoparticle marker-based biosensor using frequency mixing magnetic detection for point-of-care testing and quantification of Brucella DNA. Superparamagnetic nanoparticles were used as magnetically measured markers to selectively detect the target DNA hybridized with its complementary capture probes immobilized on a porous polyethylene filter. Experimental conditions like density and length of the probes, hybridization time and temperature, and magnetic binding specificity, sensitivity, and detection limit were investigated and optimized. Our sensor demonstrated a relatively fast detection time of approximately 10 min, with a detection limit of 55 copies (0.09 fM) when tested using DNA amplified from Brucella genetic material. In addition, the detection specificity was examined using gDNA from Brucella and other zoonotic bacteria that may coexist in the same niche, confirming the method's selectivity for Brucella DNA. Our proposed biosensor has the potential to be used for the early detection of Brucella bacteria in the field and can contribute to disease control measures.

Exploiting epoxy-rich poly(ethylene glycol) films for highly selective ssDNA sensing via electrochemical impedance spectroscopy

This study introduces an alternative approach to immobilize thiolated single-stranded DNA (ssDNA) for the DNA sensing. In contrast to the standard, monomolecular assembly of such moieties on gold substrate, over the thiolate-gold anchors, we propose to use bioinert, porous polyethylene glycol (PEG) films as a 3D template for ssDNA immobilization. The latter process relies on the reaction between the thiol group of the respectively decorated ssDNA and the epoxy groups in the epoxy-rich PEG matrix. The immobilization process and subsequent hybridization ability of the resulting sensing assembly were monitored using cyclic voltammetry and electrochemical impedance spectroscopy, with the latter tool proving itself as the most suitable transduction technique. Electrochemical data confirmed the successful immobilization of thiol-decorated ssDNA probes into the PEG matrix over the thiol-epoxy linkage as well as high hybridization efficiency, selectivity, and sensitivity of the resulting DNA sensor. Whereas this sensor was equivalent to the direct ssDNA assembly in terms of the efficiency, it exhibited a better selectivity and bioinert properties in view of the bioinert character of the PEG matrix. The above findings place PEG films as a promising platform for highly selective ssDNA sensing, leveraging their flexible chemistry, 3D character, and bioinert properties.

Recent advancements in design of nucleic acid nanocarriers for controlled drug delivery

Research of nanoscale nucleic acid carriers has garnered attention in recent years due to their distinctive and controllable properties. However, current knowledge is limited in how we can efficiently utilize these systems for clinical applications. Several researchers have pioneered new and innovative nanocarrier drug delivery systems, but understanding physiochemical properties and behavior in vivo is vital to implementing them as clinical drug delivery platforms. In this review, we outline the most significant innovations in the synthesis, physical properties, and utilization of nucleic acid nanocarriers in the past 5 years, addressing the crucial properties which improve nanocarrier characteristics, delivery, and drug release. The challenges of controlling the transport of nucleic acid nanocarriers and therapeutic release for biological applications are outlined. Barriers which inhibit effective transport into tissue are discussed with emphasis on the modifications needed to overcome such obstacles. The novel strategies discussed in this work summarize the pivotal features of modern nucleic nanocarriers and postulate where future developments could revolutionize the translation of these tools into a clinical setting.

Redox-Controlled Energy Transfer Quenching of Fluorophore-Labeled DNA SAMs Enables In Situ Study of These Complex Electrochemical Interfaces

Interfaces modified by a molecular monolayer can be challenging to study, particularly in situ, requiring novel approaches. Coupling electrochemical and optical approaches can be useful when signals are correlated. Here we detail a methodology that uses redox electrochemistry to control surface-based fluorescence intensity for detecting DNA hybridization and studying the uniformity of the surface response. A mixed composition single-strand DNA SAM was prepared using potential-assisted thiol exchange with two alkylthiol-modified ssDNAs that were either labeled with a fluorophore (AlexaFluor488) or a methylene blue (MB) redox tag. A significant change in fluorescence was observed when reducing MB to colorless leuco-MB. In situ fluorescence microscopy on a single-crystal gold bead electrode showed that fluorescence intensity depended on (1) the potential controlling the oxidation state of MB, (2) the surface density of DNA, (3) the MB:AlexFluor488 ratio in the DNA SAM, and (4) the local environment around the DNA SAM. MB efficiently quenched AlexaFluor488 fluorescence. Reduction of MB showed a significant increase in fluorescence resulting from a decrease in quenching or energy transfer efficiency. Hybridization of DNA SAMs with its unlabeled complement showed a large increase in fluorescence due to MB reduction for surfaces with sufficient DNA coverage. Comparing electrochemical-fluorescence measurements to electrochemical (SWV) measurements showed an improvement in detection of a small fraction of hybridized DNA SAM for surfaces with optimal DNA SAM composition and coverage. Additionally, this coupled electrochemical redox-fluorescence microscopy method can measure the spatial heterogeneity of electron-transfer kinetics and the influence of the local interfacial environment.

Rational Design of Porous Poly(ethylene glycol) Films as a Matrix for ssDNA Immobilization and Hybridization

Poly(ethylene glycol) (PEG) films, fabricated by thermally induced crosslinking of amine- and epoxy-terminated four-arm STAR-PEG precursors, were used as porous and bioinert matrix for single-stranded DNA (ssDNA) immobilization and hybridization. The immobilization relied on the reaction between the amine groups in the films and N-hydroxy succinimide (NHS) ester groups of the NHS-ester-decorated ssDNA. Whereas the amount of reactive amine groups in the films with the standard 1:1 composition of the precursors turned out to be too low for efficient immobilization, it could be increased noticeably using an excess (2:1) concentration of the amine-terminated precursor. The respective films retained the bioinertness of the 1:1 prototype and could be successfully decorated with probe ssDNA, resulting in porous, 3D PEG-ssDNA sensing assemblies. These assemblies exhibited high selectivity with respect to the target ssDNA strands, with a hybridization efficiency of 78–89% for the matching sequences and full inertness for non-complementary strands. The respective strategy can be applied to the fabrication of DNA microarrays and DNA sensors. As a suitable transduction technique, requiring no ssDNA labeling and showing high sensitivity in the PEG-ssDNA case, electrochemical impedance spectroscopy is suggested.

Reverse Electrochemical Sensing of FLT3-ITD Mutations in Acute Myeloid Leukemia Using Gold Sputtered ZnO-Nanorod Configured DNA Biosensors

Detection of genetic mutations leading to hematological malignancies is a key factor in the early diagnosis of acute myeloid leukemia (AML). FLT3-ITD mutations are an alarming gene defect found commonly in AML patients associated with high cases of leukemia and low survival rates. Available diagnostic assessments for FLT3-ITD are incapable of combining cost-effective detection platforms with high analytical performances. To circumvent this, we developed an efficient DNA biosensor for the recognition of AML caused by FLT3-ITD mutation utilizing electrochemical impedance characterization. The system was designed by adhering gold-sputtered zinc oxide (ZnO) nanorods onto interdigitated electrode (IDE) sensor chips. The sensing surface was biointerfaced with capture probes designed to hybridize with unmutated FLT3 sequences instead of the mutated FLT3-ITD gene, establishing a reverse manner of target detection. The developed biosensor demonstrated specific detection of mutated FLT3 genes, with high levels of sensitivity in response to analyte concentrations as low as 1 nM. The sensor also exhibited a stable functional life span of more than five weeks with good reproducibility and high discriminatory properties against FLT3 gene targets. Hence, the developed sensor is a promising tool for rapid and low-cost diagnostic applications relevant to the clinical prognosis of AML stemming from FLT3-ITD mutations.

Tailored Nucleic Acid Architectures at Gold Surfaces for Controlled Therapeutic Release

Nucleic acids are versatile materials capable of forming smart nanocarriers with highly controllable therapeutic delivery. DNA-gated release is a mechanism by which DNA oligonucleotides physically block the release of encapsulated drugs from porous nanoparticles. We extend this mechanism to be used with drugs bound to the surface of DNA-capped gold nanoparticles (AuNPs). We investigated DNA monolayers of different thicknesses and hybridization states to determine how DNA surface architecture can affect the release of a template drug bound to the gold surface. DNA layers are investigated on the planar gold surface via quartz crystal microbalance with dissipation and on AuNPs via dynamic light scattering. The resultant layer architectures were studied for their effect on the release rate of drugs. We observed that varying DNA architectures on AuNPs result in different release rates of the drug. The rate of drug release can be slowed using either folded or randomly coiled DNA sequences, which act as a physical barrier to diffusion. DNA monolayers with upright orientation release drugs more quickly. When the longer single-stranded DNA is used, the drug release is slowed even further. However, even upright DNA layers provide a barrier to drug diffusion at longer sequence lengths. We hypothesize that it is the architecture of the DNA layer, influenced by the folded or upright orientation of individual DNA molecules, that affects the free diffusion of the drug away from the AuNP surface. This mechanism may improve the biological availability of many surface-bound drugs on solid, DNA-capped nanoparticles.

Assays to Estimate the Binding Affinity of Aptamers

Aptamers have become coming-of-age molecular recognition elements in both diagnostic and therapeutic applications. Generated by SELEX, the 'quality control' of aptamers, which involves the validation of their binding affinity against their respective targets is pivotal to ascertain their potency prior to use in any downstream assays or applications. Several aptamers have been isolated thus far, however, the usage of inappropriate validation assays renders some of these aptamers dubitable in terms of their binding capabilities. Driven by this need, we provide an up-to-date critical review of the various strategies used to determine the aptamer-target binding affinity with the aim of providing researchers a better comprehension of the different analytical approaches in respect to the molecular properties of aptamers and their intended targets. The techniques reported have been classified as label-based techniques such as fluorescence intensity, fluorescence anisotropy, filter-binding assays, gel shift assays, ELISA; and label-free techniques such as UV-Vis spectroscopy, circular dichroism, isothermal titration calorimetry, native electrospray ionization-mass spectrometry, quartz crystal microbalance, surface plasmon resonance, NECEEM, backscattering interferometry, capillary electrophoresis, HPLC, and nanoparticle aggregation assays. Hybrid strategies combining the characteristics of both categories such as microscale thermophoresis have been also additionally emphasized. The fundamental principles, complexity, benefits, and challenges under each technique are elaborated in detail.

Electrochemical Beacon Method to Quantify 10 Attomolar Nucleic Acids with a Semilog Dynamic Range of 7 Orders of Magnitude

Change in the dynamics of single-stranded DNA or RNA probes tethered to an Au electrode on immunospecific binding to the analyte is a versatile approach to quantify a variety of molecules, such as heavy metal ions, pesticides, proteins, and nucleic acids (NAs). A widely studied approach is the electrochemical beacon method where the redox of a dye attached to the probe decreases as its proximity to the underlying electrode changes on binding. The limit of quantification (LOQ) defined by the semilog dependence of the signal on target concentration is in the picomolar range. Here, a method was studied where, by differential reflectivity, multiple reactions were measured on a monolith electrode. An alternative contrast mechanism was discovered, which led to an approach to enhance the LOQ to 10 aM and increase the dynamic range to 7 orders of magnitude using similar probes and binding conditions. Quantitative analysis on sequences with the G-C fraction ranging from 37 to 72% was performed. The approach will allow for the development of a label-free, enzyme-free microarray to detect biomolecules including NAs and proteins on a single electrode at quantification from 10 aM to 0.1 nM with high specificity.

Detection of necrotrophic DNA marker of anthracnose causing Colletotrichum gloeosporioides fungi in harvested produce using surface plasmon resonance

Agriculture and food crops monitoring is extremely important for securing the food supply chain to human society. Here, we developed a highly specific detection method for monitoring pathogenic fungus Colletotrichum gloeosporioides using necrotrophic DNA biomarker as the recognition element and surface plasmon resonance (SPR) as transducing mechanism in the prism coupling configuration. The sensor shows its response for a wide range of concentrations from pM to μM of target DNA sequence using a complementary DNA probe immobilized on the sensor surface, which could detect concentrations as low as 7 pM. The detection limit is found to be comparable with conventional molecular-based detection platforms, achieved due to optimized spectral SPR bimetallic substrate with subpixel resolution obtained by post processing. The response time of the sensor for detection is less than 30 min at room temperature. The quick detection scheme of the sensor may facilitate the screening of a large number of samples acquired for the sorting of harvested produce. This sensor is fast, reliable, cost-effective, and can be miniaturized for portability for the screening of real samples (mRNA) in the field and packaging house.

Nanometer-Scale Force Profiles of Short Single- and Double-Stranded DNA Molecules on a Gold Surface Measured Using a Surface Forces Apparatus

Using a surface forces apparatus (SFA), we have studied the nanomechanical behavior of short single-stranded and partially and fully double-stranded DNA molecules attached via one end to a self-assembled monolayer on a gold surface. Our results confirm the previously proposed "mushroom-like" polymer structure for surface-attached, single-stranded DNA at low packing density and a "brush-like" structure for the same construct at higher density. At low density we observe a transition to "rigid rod" behavior upon addition of DNA complementary to the surface-attached single strand as the fraction of molecules that are double-stranded increases, with a concomitant increase in the SFA-observed thickness of the monolayer and the characteristic length of the observed repulsive forces. At higher densities, in contrast, this transition is effectively eliminated, presumably because the single-stranded state is already extended in its "brush" state. Taken together, these studies offer insights into the structure and physics of surface-attached short DNAs, providing new guidance for the rational design of DNA-modified functional surfaces.

In Situ Monitoring of Rolling Circle Amplification on a Solid Support by Surface Plasmon Resonance and Optical Waveguide Spectroscopy

The growth of surface-attached single-stranded deoxyribonucleic acid (ssDNA) chains is monitored in situ using an evanescent wave optical biosensor that combines surface plasmon resonance (SPR) and optical waveguide spectroscopy (OWS). The "grafting-from" growth of ssDNA chains is facilitated by rolling circle amplification (RCA), and the gradual prolongation of ssDNA chains anchored to a gold sensor surface is optically tracked in time. At a sufficient density of the polymer chains, the ssDNA takes on a brush architecture with a thickness exceeding 10 μm, supporting a spectrum of guided optical waves traveling along the metallic sensor surface. The simultaneous probing of this interface with the confined optical field of surface plasmons and additional more delocalized dielectric optical waveguide modes enables accurate in situ measurement of the ssDNA brush thickness, polymer volume content, and density gradients. We report for the first time on the utilization of the SPR/OWS technique for the measurement of the RCA speed on a solid surface that can be compared to that in bulk solutions. In addition, the control of ssDNA brush properties by changing the grafting density and ionic strength and post-modification via affinity reaction with complementary short ssDNA staples is discussed. These observations may provide important leads for tailoring RCA toward sensitive and rapid assays in affinity-based biosensors.

Real-time monitoring of bead-based DNA hybridization in a microfluidic system: study of amplicon hybridization behavior on solid supports

DNA hybridization phenomena occurring on solid supports are not understood as clearly as aqueous phase hybridizations and mathematical models cannot predict some empirically obtained results. Ongoing research has identified important parameters but remains incomplete to accurately account for all interactions. It has previously been shown that the length of the overhanging (dangling) end of the target DNA strand following hybridization to the capture probe is correlated to interactions with the complementary strand in solution which can result in unbinding of the target and its release from the surface. We have developed an instrument for real-time monitoring of DNA hybridization on spherical particles functionalized with oligonucleotide capture probes and arranged in the form of a tightly packed monolayer bead bed inside a microfluidic cartridge. The instrument is equipped with a pneumatic module to mediate displacement of fluid on the cartridge. We compared this system to both conventional (passive) and centrifugally-driven (active) microfluidic microarray hybridization on glass slides to establish performance levels for the detection of single nucleotide polymorphisms. The system was also used to study the effect of the dangling end's length in real-time when the immobilized target DNA is exposed to the complementary strand in solution. Our findings indicate that increasing the length of the dangling end leads to desorption of target amplicons from bead-bound capture probes at a rate approaching that of the initial hybridization process. Finally, bead bed hybridization was performed with Streptococcus agalactiae cfb gene amplicons obtained from randomized clinical samples, which allowed for identification of group B streptococci within 5-15 min. The methodology presented here is useful for investigating competitive hybridization mechanisms on solid supports and to rapidly validate the suitability of microarray capture probes.

Novel electro self-assembled DNA nanospheres as a drug delivery system for atenolol

We describe new method for preparing DNA nanospheres for a self-assembled atenolol@DNA (core/shell) drug delivery system. In this paper, we propose the electrochemical transformation of an alkaline polyelectrolyte solution of DNA into DNA nanospheres. We successfully electrosynthesized DNA nanospheres that were stable for at least 2 months at 4 °C. UV-visible spectra of the prepared nanospheres revealed a peak ranging from 372 to 392 nm depending on the DNA concentration and from 361 to 398.3 nm depending on the electrospherization time. This result, confirmed with size distribution curves worked out from transmission electron microscopy (TEM) images, showed that increasing electrospherization time (6, 12 and 24 h) induces an increase in the average size of DNA nanospheres (48, 65.5 and 117 nm, respectively). In addition, the average size of DNA nanospheres becomes larger (37.8, 48 and 76.5 nm) with increasing DNA concentration (0.05, 0.1 and 0.2 wt%, respectively). Also, the affinity of DNA chains for the surrounding solvent molecules changed from favorable to bad with concomitant extreme reduction in the zeta potential from -31 mV to -17 mV. Principally, the attractive and hydrophobic interactions tend to compact the DNA chain into a globule, as confirmed by Fourier transform infrared spectroscopy (FTIR) and TEM. To advance possible applications, we successfully electro self-assembled an atenolol@DNA drug delivery system. Our findings showed that electrospherization as a cost-benefit technique could be effectively employed for sustained drug release. This delivery system achieved a high entrapment efficiency of 68.03 ± 2.7% and a moderate drug-loading efficiency of 3.73%. The FTIR spectra verified the absence of any chemical interaction between the drug and the DNA during the electrospherization process. X-ray diffraction analysis indicated noteworthy lessening in atenolol crystallinity. The present findings could aid the effectiveness of electrospherized DNA for use in various other pharmaceutical and biomedical applications.

The Significance of Nonlinear Screening and the pH Interference Mechanism in Field-Effect Transistor Molecular Sensors

Electrolyte screening is well known for its detrimental impact on the sensitivity of liquid-gated field-effect transistor (FET) molecular sensors and is mostly described by the linearized Debye-Hückel model. However, charged and pH-sensitive FET sensing surfaces can limit the FET molecular sensitivity beyond the Debye-Hückel screening formalism. Pre-existing surface charges can lead to the breakdown of Debye-Hückel screening and induce enhanced nonlinear Poisson-Boltzmann screening. Moreover, the charging of the pH-sensitive surface groups interferes with biomolecule sensing resulting in a pH interference mechanism. With analytical equations and TCAD simulations, we highlight that the Debye-Hückel approximation can underestimate screening and overestimate FET molecular sensitivity by more than an order of magnitude. Screening strengthens significantly beyond Debye-Hückel in the proximity of even moderately charged surfaces and biomolecule charge densities (≥1 × 10¹² q/cm²). We experimentally show the strong impact of both nonlinear screening and the pH interference effect on charge-based biomolecular sensing using a model system based on the covalent binding of single-stranded DNA on silicon FET sensors. The DNA signal increases from 24 mV at pH 7 to 96 mV at pH 3 in 1.5 mM PBS for a DNA density of 7 × 10¹² DNA/cm². Our model quantitatively explains the signal's pH dependence with roughly equal nonlinear screening and pH interference contributions. This work shows the importance of reducing the net charge and the pH sensitivity of the sensing surface to improve molecular sensing. Therefore, tailoring the gate dielectric and functional layer of FET sensors is a promising route to strong silicon FET molecular sensitivity boosts.

How Surfaces Affect Hybridization Kinetics

Hybridization between nucleic acid strands immobilized on a solid support with partners in solution is widely practiced in bioanalytical technologies and materials science. An important fundamental aspect of understanding these reactions is the role played by immobilization in the dynamics of duplex formation and disassembly. This report reviews and analyzes literature kinetic data to identify commonly observed trends and to correlate them with probable molecular mechanisms. The analysis reveals that while under certain conditions impacts from immobilization are minimal so that surface and solution hybridization kinetics are comparable, it is more typical to observe pronounced offsets between the two scenarios. In the forward (hybridization) direction, rates at the surface commonly decrease by one to two decades relative to solution, while in the reverse direction rates of strand separation at the surface can exceed those in solution by tens of decades. By recasting the deviations in terms of activation barriers, a consensus of how immobilization impacts nucleation, zipping, and strand separation can be conceived within the classical mechanism in which duplex formation is rate limited by preassembly of a nucleus a few base pairs in length, while dehybridization requires the cumulative breakup of base pairs along the length of a duplex. Evidence is considered for how excess interactions encountered on solid supports impact these processes.

Tumor-Targeting Cholesterol-Decorated DNA Nanoflowers for Intracellular Ratiometric Aptasensing

Probing endogenous molecular profiles is of fundamental importance to understand cellular function and processes. Despite the promise of programmable nucleic-acid-based aptasensors across the breadth of biomolecular detection, target-responsive aptasensors enabling intracellular detection are as of yet infrequently realized. Several challenges remain, including the difficulties in quantification/normalization of quencher-based intensiometric signals, stability issues of the probe architecture, and complex sensor operations often necessitating extensive structural modeling. Here, the biomimetic crystallization-empowered self-assembly of a tumor-targetable DNA-inorganic hybrid nanocomposite aptasensor is presented, which enables Förster resonance energy transfer (FRET)-based quantitative interpretation of changes in the cellular target abundance. Leveraging the design programmability and high-throughput fabrication of rolling circle amplification-driven DNA nanoarchitecture, this designer platform offers a method to self-assemble a robust nanosensor from a multifunctionality-encoded template that includes a cell-targeting aptamer, a ratiometric aptasensor, and a cholesterol-decorating element. Taking prostate cancer cells and intracellular adenosine triphosphate molecules as a model system, a synergistic effect in the targeted delivery by cholesterol and aptamers, and the feasibility of quantitative intracellular aptasensing are demonstrated. It is envisioned that this approach provides a highly generalizable strategy across wide-ranging target systems toward a biologically deliverable nanosensor that enables quantitative monitoring of the abundance of endogenous biomolecules.

Toward a Quantitative Relationship between Nanoscale Spatial Organization and Hybridization Kinetics of Surface Immobilized Hairpin DNA Probes

Hybridization of DNA probes immobilized on a solid support is a key process for DNA biosensors and microarrays. Although the surface environment is known to influence the kinetics of DNA hybridization, so far it has not been possible to quantitatively predict how hybridization kinetics is influenced by the complex interactions of the surface environment. Using spatial statistical analysis of probes and hybridized target molecules on a few electrochemical DNA (E-DNA) sensors, functioning through hybridization-induced conformational change of redox-tagged hairpin probes, we developed a phenomenological model that describes how the hybridization rates for single probe molecules are determined by the local environment. The predicted single-molecule rate constants, upon incorporation into numerical simulation, reproduced the overall kinetics of E-DNA sensor surfaces at different probe densities and different degrees of probe clustering. Our study showed that the nanoscale spatial organization is a major factor behind the counterintuitive trends in hybridization kinetics. It also highlights the importance of models that can account for heterogeneity in surface hybridization. The molecular level understanding of hybridization at surfaces and accurate prediction of hybridization kinetics may lead to new opportunities in development of more sensitive and reproducible DNA biosensors and microarrays.

Single-Step Printable Hydrogel Microarray Integrating Long-Chain DNA for the Discriminative and Size-Specific Sensing of Nucleic Acids

A simple approach to fabricating hydrogel-based DNA microarrays is reported by physically entrapping the rolling circle amplification (RCA) product inside printable in situ gelling hydrazone cross-linked poly(oligoethylene glycol methacrylate) hydrogels. The hydrogel-printed RCA microarray facilitates improved RCA immobilization (>65% even after vigorous washing) and resistance to denaturation relative to RCA-only printed microarrays in addition to size-discriminative sensing of DNA probes (herein, 27 or fewer nucleotides) depending on the internal porosity of the hydrogel. Furthermore, the high number of sequence repeats in the concatemeric RCA product enables high-sensitivity detection of complementary DNA probes without the need for signal amplification, with signal/noise ratios of 10 or more achieved over a short 30 min assay time followed by minimal washing. The inherent antifouling properties of the hydrogel enable discriminative hybridization in complex biological samples, particularly for short (∼10 nt) oligonucleotides whose hybridization in other assays tends to be transient and of low affinity. The scalable manufacturability and efficient performance of these hydrogel-printed RCA microarrays thus offer potential for rapid, parallel, and inexpensive sensing of short DNA/RNA biomarkers and ligands, a critical current challenge in diagnostic and affinity screening assays.

Effects of Experimental Conditions on the Signaling Fidelity of Impedance-Based Nucleic Acid Sensors

Electrochemical impedance spectroscopy (EIS), an extremely sensitive analytical technique, is a widely used signal transduction method for the electrochemical detection of target analytes in a broad range of applications. The use of nucleic acids (aptamers) for sequence-specific or molecular detection in electrochemical biosensor development has been extensive, and the field continues to grow. Although nucleic acid-based sensors using EIS offer exceptional sensitivity, signal fidelity is often linked to the physical and chemical properties of the electrode-solution interface. Little emphasis has been placed on the stability of nucleic acid self-assembled monolayers (SAMs) over repeated voltammetric and impedimetric analyses. We have studied the stability and performance of electrochemical biosensors with mixed SAMs of varying length thiolated nucleic acids and short mercapto alcohols on gold surfaces under repeated electrochemical interrogation. This systematic study demonstrates that signal fidelity is linked to the stability of the SAM layer and nucleic acid structure and the packing density of the nucleic acid on the surface. A decrease in packing density and structural changes of nucleic acids significantly influence the signal change observed with EIS after routine voltammetric analysis. The goal of this article is to improve our understanding of the effect of multiple factors on EIS signal response and to optimize the experimental conditions for development of sensitive and reproducible sensors. Our data demonstrate a need for rigorous control experiments to ensure that the measured change in impedance is unequivocally a result of a specific interaction between the target analyte and nucleic recognition element.

Elementary processes of DNA surface hybridization resolved by single-molecule kinetics: implication for macroscopic device performance

Direct monitoring of single-molecule reactions has recently become a promising means of mechanistic investigation. However, the resolution of reaction pathways from single-molecule experiments remains elusive, primarily because of interference from extraneous processes such as bulk diffusion. Herein, we report a single-molecule kinetic investigation of DNA hybridization on a metal surface, as an example of a bimolecular association reaction. The tip of the scanning tunneling microscope (STM) was functionalized with single-stranded DNA (ssDNA), and hybridization with its complementary strand on an Au(111) surface was detected by the increase in the electrical conductance associated with the electron transport through the resulting DNA duplex. Kinetic analyses of the conductance changes successfully resolved the elementary processes, which involve not only the ssDNA strands and their duplex but also partially hybridized intermediate strands, and we found an increase in the hybridization efficiency with increasing the concentration of DNA in contrast to the knowledge obtained previously by conventional ensemble measurements. The rate constants derived from our single-molecule studies provide a rational explanation of these findings, such as the suppression of DNA melting on surfaces with higher DNA coverage. The present methodology, which relies on intermolecular conductance measurements, can be extended to a range of single-molecule reactions and to the exploration of novel chemical syntheses.

Hybridization of a single DNA molecule on a surface was investigated by electrical conductance measurements. The hybridization efficiency increases with increasing the DNA concentration, in contrast to preceding studies with ensemble studies.

Label-Free Sensitive Detection of Steroid Hormone Cortisol Based on Target-Induced Fluorescence Quenching of Quantum Dots

We discovered that several types of steroid hormones quench the fluorescence of quantum dots (QDs) at close proximity. Inspired by the finding, we developed a new type of biosensor for the sensitive detection of cortisol via direct fluorescence quenching of functionalized QD probes directly induced by the capture of target cortisol without additional reporter reagents. The detection selectivity was provided by cortisol-selective aptamers or anticortisol antibodies conjugated on the QD surfaces. With the magnetic nanoparticle labeling, the new sensing method enabled rapid cortisol sensing at physiologically relevant concentrations and yielded the detection limit of ∼1 nM for aptamer-based sensors and ∼100 pM for antibody-based sensors. We also evaluated the new detection method using saliva samples with an optimized sample preparation process under the assistance of magnetic manipulation. The result showed a satisfying recovery rate for spiked saliva tests. The facile sensing technology offers an appealing approach for the detection of steroid hormones in point-of-care settings.

Strategies to bioengineer aptamer-driven nanovehicles as exceptional molecular tools for targeted therapeutics: A review

Aptamers are a class of folded nucleic acid strands capable of binding to different target molecules with high affinity and selectivity. Over the years, they have gained a substantial amount of interest as promising molecular tools for numerous medical applications, particularly in targeted therapeutics. However, only the different treatment approaches and current developments of aptamer-drug therapies have been discussed so far, ignoring the crucial technical and functional aspects of constructing a therapeutically effective aptamer-driven drug delivery system that translates to improved in-vivo performance. Hence, this paper provides a comprehensive review of the strategies used to improve the therapeutic performance of aptamer-guided delivery systems. We focus on the different functional features such as drug deployment, payload capacity, in-vivo stability and targeting efficiency to further our knowledge in enhancing the cell-specific delivery of aptamer-drug conjugates. Each reported strategy is critically discussed to emphasize both the benefits provided in comparison with other similar techniques and to outline their potential drawbacks with respect to the molecular properties of the aptamers, the drug and the system to be designed. The molecular architecture and design considerations for an efficient aptamer-based delivery system are also briefly elaborated.

Characterization and application of fluidic properties of trinucleotide repeat sequences by wax-on-plastic microfluidics

Trinucleotide repeat (TNR) sequences introduce sequence-directed flexibility in the genomic makeup of all living species leading to unique non-canonical structure formation. In humans, the expansions of TNR sequences are responsible for almost 24 neurodegenerative and neuromuscular diseases because their unique structures disrupt cell functions. The biophysical studies of these sequences affect their electrophoretic mobility and spectroscopic signatures. Here, we demonstrate a novel strategy to characterize and discriminate the TNR sequences by monitoring their capillary flow in the absence of an external driving force using wax-on-plastic microchannels. The wax-on-plastic microfluidic system translates the sequence-directed flexibility of TNR into differential flow dynamics. Several variables were used to characterize sequences including concentration, single- vs. double-stranded samples, type of repeat sequence, length of the repeat sequence, presence of mismatches in duplex, and presence of metal ion. All these variables were found to influence the flow velocities of TNR sequences as these factors directly affect the structural flexibility of TNR at the molecular level. An overall trend was observed as the higher flexibility in the TNR structure leads to lower capillary flow. After testing samples derived from relevant cells harboring expanded TNR sequences, it is concluded that this approach may transform into a reagent-free and pump-free biosensing platform to detect microsatellite expansion diseases.

Unique sequence-dependent properties of trinucleotide repeat monolayers: electrochemical, electrical, and topographic characterization

The sequence-dependent properties of the surface-assembled trinucleotide repeat interface on a gold surface were explored by electrochemical methods and surface probe microscopy.

The Surface Science of Microarray Generation-A Critical Inventory

Microarrays are powerful tools in biomedical research and have become indispensable for high-throughput multiplex analysis, especially for DNA and protein analysis. The basis for all microarray processing and fabrication is surface modification of a chip substrate and many different strategies to couple probe molecules to such substrates have been developed. We present here a critical assessment of typical biochip generation processes from a surface science point of view. While great progress has been made from a molecular biology point of view on the development of qualitative assays and impressive results have been obtained on the detection of rather low concentrations of DNA or proteins, quantitative chip-based assays are still comparably rare. We argue that lack of stable and reliable deposition chemistries has led in many cases to suboptimal quantitative reproducibility, impeded further progress in microarray development and prevented a more significant penetration of microarray technology into the diagnostic market. We suggest that surface-attached hydrogel networks might be a promising strategy to achieve highly sensitive and quantitatively reproducible microarrays.

Lateral and Vertical Flow Assays for Point-of-Care Diagnostics

Lateral flow assays (LFAs) have been the pillar of rapid point-of-care (POC) diagnostics due to their simplicity, rapid process, and low cost. Recent advances in sensitivity, selectivity, and chemical stability enhancement have ensured the foothold of LFAs in commercial POC diagnostics. This paper reviews recent developments in labeling strategies and detection methods of LFAs. Moreover, vertical flow assays (VFAs) have emerged as an alternate paper-based assay due to faster detection time and unique multiplexing capabilities. Smartphones as LFA readers have been transformed into a universal integrated platform for imaging, data processing, and storage, providing quantitative results in low-resource settings. Commercial LFAs and VFAs products are evaluated with regards to their performance, market trends, and regulatory issues. The future outlook of the flow-based assays for POC diagnostics is also discussed.

P and N type copper phthalocyanines as effective semiconductors in organic thin-film transistor based DNA biosensors at elevated temperatures

Many health-related diagnostics are expensive, time consuming and invasive. Organic thin film transistor (OTFT) based devices show promise to enable rapid, low cost diagnostics that are an important aspect to enabling increased access and availability to healthcare. Here, we describe OTFTs based upon two structurally similar P (copper phthalocyanine – CuPc) and N (hexdecafluoro copper phthalocyanine – F₁₆-CuPc) type semiconductor materials, and demonstrate their potential for use as both temperature and DNA sensors. Bottom gate bottom contact (BGBC) OTFTs with either CuPc or F₁₆-CuPc semiconducting layers were characterized within a temperature range of 25 °C to 90 °C in both air and under vacuum. CuPc devices showed small positive shifts in threshold voltage (V_T) in air and significant linear increases in mobility with increasing temperature. F₁₆-CuPc devices showed large negative shifts in V_T in air and linear increases in mobility under the same conditions. Similar OTFTs were exposed to DNA in different hybridization states and both series of devices showed positive V_T increases upon DNA exposure, with a larger response to single stranded DNA. The N-type F₁₆-CuPc devices showed a much greater sensing response than the P-type CuPc. These findings illustrate the use of these materials, especially the N-type semiconductor, as both temperature and DNA sensors and further elucidate the mechanism of DNA sensing in OTFTs.

This study illustrates the use of an N-type semiconductor, in both temperature and DNA sensors and further elucidates the mechanism of DNA sensing in OTFTs.

Microlens Enhancement of Surface-Tethered Molecular Beacons

The tethering of molecular beacon oligonucleotide detection probes to surface-patterned poly(ethylene glycol) (PEG) microgels has enabled the integration of molecular beacons into a microarray format. The microgels not only localize the probes to specific surface positions but also maintain them in a waterlike environment. Here we extend the concept of microgel tethering to include dielectric microlenses. We show that streptavidin-functionalized polystyrene microspheres (3 μm diameter) can be colocalized with molecular beacons using biotinylated PEG gels in patterns ranging from pseudocontinuous microgel pads with lateral dimensions on the order of tens of micrometers to individual microgels with lateral dimensions on the order of 400-500 nm. We use a simplex assay based on Influenza A detection to study the lensing behavior. The microspheres increase the effective numerical aperture of the collection optics, and we find that a tethered microsphere increases the peak intensity collected from hybridized beacons between 1.5 and 10 times depending on the specific pattern size and areal density of microgels. The highest signal increase occurs when a single microsphere is tethered to a single isolated microgel. The tethering is highly self-directed and occurs in the individual-microgel case only when the microgel is close to the optic axis of the microsphere. This alignment minimizes spherical aberration and maximizes coupling of emitted fluorescent intensity into the collection optics.

Single Molecule Profiling of Molecular Recognition at a Model Electrochemical Biosensor

The spatial arrangement of target and probe molecules on the biosensor is a key aspect of the biointerface structure that ultimately determines the properties of interfacial molecular recognition and the performance of the biosensor. However, the spatial patterns of single molecules on practical biosensors have been unknown, making it difficult to rationally engineer biosensors. Here, we have used high-resolution atomic force microscopy to map closely spaced individual probes as well as discrete hybridization events on a functioning electrochemical DNA sensor surface. We also applied spatial statistical methods to characterize the spatial patterns at the single molecule level. We observed the emergence of heterogeneous spatiotemporal patterns of surface hybridization of hairpin probes. The clustering of target capture suggests that hybridization may be enhanced by proximity of probes and targets that are about 10 nm away. The unexpected enhancement was rationalized by the complex interplay between the nanoscale spatial organization of probe molecules, the conformational changes of the probe molecules, and target binding. Such molecular level knowledge may allow one to tailor the spatial patterns of the biosensor surfaces to improve the sensitivity and reproducibility.

Specific detection of stable single nucleobase mismatch using SU-8 coated silicon nanowires platform

Novel microarray platform for single nucleotide polymorphisms (SNPs) detection has been developed using silicon nanowires (SiNWs) as support and two different surface modification methods for attaining the necessary functional groups. Accordingly, we compared the detection specificity and stability over time of the probes printed on SiNWs modified with (3-aminopropyl)triethoxysilane (APTES) and glutaraldehyde (GAD), or coated with a simpler procedure using epoxy-based SU-8 photoresist. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were used for comparative characterization of the unmodified and coated SiNWs. The hybridization efficiency was assessed by comprehensive statistical analysis of the acquired data from confocal fluorescence scanning of the manufactured biochips. The high detection specificity between the hybridized probes containing different mismatch types was demonstrated on SU-8 coating by one way ANOVA test (adjusted p value *** < .0001). The stability over time of the probes tethered on SiNWs coated with SU-8 was evaluated after 1, 4, 8 and 21 days of probe incubation, revealing values for coefficient of variation (CV) between 2.4% and 5.6%. The signal-to-both-standard-deviations ratio measured for SU-8 coated SiNWs platform was similar to the commercial support, while the APTES-GAD coated SiNWs exhibited the highest values.

Solid-Phase Nucleic Acid Sequence-Based Amplification and Length-Scale Effects during RNA Amplification

Solid-phase oligonucleotide amplification is of interest because of possible applications to next-generation sequencing, multiplexed microarray-based detection, and cell-free synthetic biology. Its efficiency is, however, less than that of traditional liquid-phase amplification involving unconstrained primers and enzymes, and understanding how to optimize the solid-phase amplification process remains challenging. Here, we demonstrate the concept of solid-phase nucleic acid sequence-based amplification (SP-NASBA) and use it to study the effect of tethering density on amplification efficiency. SP-NASBA involves two enzymes, avian myeloblastosis virus reverse transcriptase (AMV-RT) and RNase H, to convert tethered forward and reverse primers into tethered double-stranded DNA (ds-DNA) bridges from which RNA^- amplicons can be generated by a third enzyme, T7 RNA polymerase. We create microgels on silicon surfaces using electron-beam patterning of thin-film blends of hydroxyl-terminated and biotin-terminated poly(ethylene glycol) (PEG-OH, PEG-B). The tethering density is linearly related to the PEG-B concentration, and biotinylated primers and molecular beacon detection probes are tethered to streptavidin-activated microgels. While SP-NASBA is very efficient at low tethering densities, the efficiency decreases dramatically with increasing tethering density due to three effects: (a) a reduced hybridization efficiency of tethered molecular beacon detection probes; (b) a decrease in T7 RNA polymerase efficiency;

Use of a redox probe for an electrochemical RNA-ligand binding assay in microliter droplets

In this work, we report an affordable, sensitive, fast and user-friendly electroanalytical method for monitoring the binding between unlabeled RNA and small compounds in microliter-size droplets using a redox-probe and disposable miniaturized screen-printed electrochemical cells.

Target Confinement in Small Reaction Volumes Using Microfluidic Technologies: A Smart Approach for Single-Entity Detection and Analysis

Over the last decades, the study of cells, nucleic acid molecules, and proteins has evolved from ensemble measurements to so-called single-entity studies. The latter offers huge benefits, not only as biological research tools to examine heterogeneities among individual entities within a population, but also as biosensing tools for medical diagnostics, which can reach the ultimate sensitivity by detecting single targets. Whereas various techniques for single-entity detection have been reported, this review focuses on microfluidic systems that physically confine single targets in small reaction volumes. We categorize these techniques as droplet-, microchamber-, and nanostructure-based and provide an overview of their implementation for studying single cells, nucleic acids, and proteins. We furthermore reflect on the advantages and limitations of these techniques and highlight future opportunities in the field.

Molecular conformations of DNA targets captured by model nanoarrays

An open question in single molecule nanoarrays is how the chemical and morphological heterogeneities of the solid support affect the properties of biomacromolecules. We generated arrays that allowed individually-resolvable DNA molecules to interact with tailored surface heterogeneities and revealed how molecular conformations are impacted by surface interactions.

Stretching of surface-tethered polymers in pressure-driven flow under confinement

We study the effect of pressure-driven flow on a single surface-tethered DNA molecule confined between parallel surfaces. The influence of flow and channel parameters as well as the length of the molecules on their extension and orientation is explored. In the experiments the chain conformations are imaged by laser scanning confocal microscopy. We find that the fractional extension of the tethered DNA molecules mainly depends on the wall shear stress, with effects of confinement being very weak. Experiments performed with molecules of different contour length show that the fractional extension is a universal function of the product of the wall shear stress and the contour length, a result that can be obtained from a simple scaling relation. The experimental results are in good agreement with results from coarse-grained molecular dynamics/Lattice-Boltzmann simulations.

Electric-field-induced stretching of surface-tethered polyelectrolytes in a microchannel

We study the stretching of a surface-tethered polyelectrolyte confined between parallel surfaces under the application of a dc electric field. We explore the influence of the electric-field strength, the length of the polyelectrolyte, and the degree of confinement on the conformation of the polyelectrolyte by single-molecule experiments and coarse-grained coupled lattice-Boltzmann molecular-dynamics simulations. The fractional extension of the polyelectrolyte is found to be a universal function of the product of the applied electric field and the molecular contour length, which is explained by simple scaling arguments. The degree of confinement does not have any significant influence on the stretching. We also confirm that an electrohydrodynamic equivalence principle relating the stretching in an electric field to that in a flow field is applicable.

Interfacial adsorption of pH-responsive polymers and nanoparticles

Using dissipative particle dynamics (DPD), we model the interfacial adsorption of pH-responsive polyelectrolytes and polyelectrolyte-grafted nanoparticles (PNPs) at a planar water-oil interface. The electrostatic interactions in the presence of the dielectric discontinuity across the interface are modeled by exploiting the Groot method, which uses an iterative method to solve the Poisson equation on a uniform grid with distributed charge. We reveal the effects of the pH and salinity of the aqueous solution and the length of the polyelectrolyte on the adsorption behavior of weak polyelectrolytes. The adsorption kinetics is monitored via the trajectory of the center of mass of the polyelectrolyte in the direction normal to the interface. The residence time at the interface and the pair correlation function between the polyelectrolyte and the oil are measured to quantitatively characterize the adsorption. Similar to the weak polyelectrolytes, the influences of pH, salinity and grafted chain length on the adsorption of an individual PNP are explored. Our results show that by grafting polyelectrolytes, the interfacial behavior of the nanoparticles can be tuned by changing the pH and salinity of the solution, which is dictated by the contact angle, the pair correlation function between the particles and the oil, the desorption energy, and the particle morphology at the interface. We also observe that the electrostatic-driven variations in the interfacial activity and morphology of the PNPs are not sensitive to the length of the grafted polyelectrolytes.

Ionic strength-controlled hybridization and stability of hybrids of KRAS DNA single-nucleotides: A surface plasmon resonance study

The discrimination of a fully matched, unlabeled KRAS wild-type (WT) (C-G) target sample with respect to three of the most frequent KRAS codon mutations (G12 S (C-A), G12 R (C-C), G12C (C-T)) was investigated using an optimized detection strategy involving surface plasmon resonance (SPR), based on optimized probe-surface density and ionic strength control. The changes observed in the SPR signal were always larger for WT compared with the single-mismatch target DNA oligonucleotides, and were aligned with the theoretical energy differences between the base pair C-G, C-T, C-A, C-C. Hybridization rates of ∼10⁶M^-1s^-1 were detected without the introduction of high temperature and labels, usually needed in conventional hybridization methods. One hundred percent mutation discrimination of the matched KRAS wild-type (C-G) sequence with respect to three mismatched G12C (C-T), G12 S (C-A), G12 R (C-C) target sequences was achieved.