Coffee ring aptasensor for rapid protein detection

Fundamentals and Applications of Surface Wetting

In an era defined by an insatiable thirst for sustainable energy solutions, responsible water management, and cutting-edge lab-on-a-chip diagnostics, surface wettability plays a pivotal role in these fields. The seamless integration of fundamental research and the following demonstration of applications on these groundbreaking technologies hinges on manipulating fluid through surface wettability, significantly optimizing performance, enhancing efficiency, and advancing overall sustainability. This Review explores the behavior of liquids when they engage with engineered surfaces, delving into the far-reaching implications of these interactions in various applications. Specifically, we explore surface wetting, dissecting it into three distinctive facets. First, we delve into the fundamental principles that underpin surface wetting. Next, we navigate the intricate liquid-surface interactions, unraveling the complex interplay of various fluid dynamics, as well as heat- and mass-transport mechanisms. Finally, we report on the practical realm, where we scrutinize the myriad applications of these principles in everyday processes and real-world scenarios.

Soluto-thermal Marangoni convection in stationary micro-bioreactors on heated substrates: Tool for in vitro diagnosis of PSA

The investigation of antigen-laden droplet deposition patterns on antibody-immobilized substrates has potential for disease detection. Stationary droplets that contain antigens on surfaces immobilized with antibodies can function as microreactors. Temperature modulation enhances reaction efficiency and reduces detection time in droplet-based systems. Thus, the aim of this study is to explore the impact of substrate heating on the structures of protein deposits and the influence of substrate temperature on thermo-solutal Marangoni convection within the droplets. Previous research has explored deposition patterns as diagnostic tools, but limited investigations have focused on the effects of substrate heating on protein deposit structures and the influence of substrate temperature on thermo-solutal Marangoni convection within droplets, creating a knowledge gap. In this study, we conducted experiments to explore how heating the substrate affects the deposition patterns of droplets containing prostate-specific antigen (PSA) on a substrate immobilized with anti-PSA IgG. Additionally, we investigated the thermo-solutal Marangoni convection within these droplets. Our findings reveal distinct deposition patterns classified into dendritic structures (heterogeneous), transitional patterns, and needle-like (homogeneous) structures. The presence of prominent coffee rings and the variation in crystal size across different groups highlight the interplay between thermal and solutal Marangoni advection. Entropy analysis provides insights into structural differences within and between patterns. This work optimizes substrate temperatures for reduced evaporation and detection times while preserving protein integrity, advancing diagnostic tool development, and improving understanding of droplet-based systems.

Drying behaviour of nanofluid sessile droplets on self-affine vis-à-vis corrugated nanorough surfaces

In recent years, evaporative self-assembly of sessile droplets has gained considerable attention owing to its wide applicability in many areas. While the phenomenon is well studied for smooth and isotropically rough (self-affine) surfaces, investigations comparing the outcomes on self-affine vis-à-vis corrugated surfaces remains to be done. In this experimental work, we compare the wetting and evaporation dynamics of nano-colloidal microlitre droplets on self-affine and corrugated nanorough surfaces having identical roughnesses and interface properties. The coupled influence of particle size, concentration, and surface structuring has been explored. Differences in wettability and evaporation dynamics are observed, which are explained via the interaction between wetting fluid and anisotropic surface roughness. Our findings exhibit different temporal behaviour of contact radius and angle in the evaporation process of the droplets. Further, the corrugated surface exhibits anisotropic wettability with a monotonic change in droplet shape as evaporation proceeds, finally giving rise to irregular dried patterns. The scaled rim width and crack spacing of the particulate deposits are examined. Our results can inspire fabrication of surfaces that can facilitate direction-dependent droplet motion for specific applications.

Development of a Low-Cost Paper-Based Platform for Coffee Ring-Assisted SERS

The need for highly sensitive, low-cost, and timely diagnostic technologies at the point of care is increasing. Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopic technique that is an advantageous technique to address this need, as it can rapidly detect analytes in small or dilute samples with improved sensitivity compared to conventional Raman spectroscopy. Despite the many advantages of SERS, one drawback of the technique is poor reproducibility due to variable interactions between nanoparticles and target analytes. To overcome this limitation, coupling SERS with the coffee ring effect has been implemented to concentrate and localize analyte-nanoparticle conjugates for improved signal reproducibility. However, current coffee ring platforms require laborious fabrication steps. Herein, we present a low-cost, two-step fabrication process for coffee ring-assisted SERS, utilizing wax-printed nitrocellulose paper. The platform was designed to produce a highly hydrophobic paper substrate that supports the coffee ring effect and tested using gold nanoparticles for SERS sensing. The nanoparticle concentration and solvent were varied to determine the effect of solution composition on ring formation and center clearance. The SERS signal was validated using 4-mercaptobenzoic acid (MBA) and tested with Moraxella catarrhalis bacteria to ensure functionality for chemical and biological applications. The limit of detection using MBA is 41.56 nM, and the biochemical components of the bacterial cell wall were enhanced with low spectral variability. The developed platform is advantageous due to ease of fabrication and use, representing the next step toward implementing low-cost coffee ring-assisted SERS for point-of-care sensing.

Simultaneous quantification of serum albumin and gamma globulin using Zn(II)-metallosurfactant via a coffee ring pattern

Herein, we report interactivity and conjugate formation ability between a Zn(II)-metallosurfactant and two clinically relevant serum proteins, albumin (ALB) and γ-globulin (GGB). We found that the surfactant-ALB conjugate promotes coffee ring formation, whereas with GGB it gets suppressed, which is due to the difference in structural anisotropy and hydrophobicity of the conjugates. Additionally, validation of this biosensing platform has been established in human serum samples, and it has potential applications for on-spot rapid diagnostics in remote areas.

The control of dry-out patterns using bubble-containing droplets

HYPOTHESIS

For an evaporating nanofluid droplet that contains a bubble inside, we suspect the bubble boundary remains pinned during evaporation whereas the droplet perimeter recedes. Thus, the dry-out patterns are mainly determined by the presence of the bubble and their morphology can be tuned by the size and location of the added bubble.

EXPERIMENTS

Bubbles with varying base diameters and lifetimes are added into evaporating droplets that contain nanoparticles with different types, sizes, concentrations, shapes, and wettability. The geometric dimensions of the dry-out patterns are measured.

FINDINGS

For a droplet containing a long-lifetime bubble, a complete ring-like deposit forms, and its diameter and thickness increases and decreases with the bubble base diameter, respectively. The ring completeness, i.e., the ratio of actual ring length to its imaginary perimeter, decreases with the decrease in bubble lifetime. The pinning of droplet receding contact line by particles near the bubble perimeter has been found to be the key factor leading to ring-like deposits. This study introduces a strategy of producing ring-like deposit and allows a control of the ring morphology in a simple, cheap, and impurity-free fashion, which is applicable to various applications associated with evaporative self-assembly.

Drying of bio-colloidal sessile droplets: Advances, applications, and perspectives

Drying of biologically-relevant sessile droplets, including passive systems such as DNA, proteins, plasma, and blood, as well as active microbial systems comprising bacterial and algal dispersions, has garnered considerable attention over the last decades. Distinct morphological patterns emerge when bio-colloids undergo evaporative drying, with significant potential in a wide range of biomedical applications, spanning bio-sensing, medical diagnostics, drug delivery, and antimicrobial resistance. Consequently, the prospects of novel and thrifty bio-medical toolkits based on drying bio-colloids have driven tremendous progress in the science of morphological patterns and advanced quantitative image-based analysis. This review presents a comprehensive overview of bio-colloidal droplets drying on solid substrates, focusing on the experimental progress during the last ten years. We provide a summary of the physical and material properties of relevant bio-colloids and link their native composition (constituent particles, solvent, and concentrations) to the patterns emerging due to drying. We specifically examined the drying patterns generated by passive bio-colloids (e.g., DNA, globular, fibrous, composite proteins, plasma, serum, blood, urine, tears, and saliva). This article highlights how the emerging morphological patterns are influenced by the nature of the biological entities and the solvent, micro- and global environmental conditions (temperature and relative humidity), and substrate attributes like wettability. Crucially, correlations between emergent patterns and the initial droplet compositions enable the detection of potential clinical abnormalities when compared with the patterns of drying droplets of healthy control samples, offering a blueprint for the diagnosis of the type and stage of a specific disease (or disorder). Recent experimental investigations of pattern formation in the bio-mimetic and salivary drying droplets in the context of COVID-19 are also presented. We further summarized the role of biologically active agents in the drying process, including bacteria, algae, spermatozoa, and nematodes, and discussed the coupling between self-propulsion and hydrodynamics during the drying process. We wrap up the review by highlighting the role of cross-scale in situ experimental techniques for quantifying sub-micron to micro-scale features and the critical role of cross-disciplinary approaches (e.g., experimental and image processing techniques with machine learning algorithms) to quantify and predict the drying-induced features. We conclude the review with a perspective on the next generation of research and applications based on drying droplets, ultimately enabling innovative solutions and quantitative tools to investigate this exciting interface of physics, biology, data sciences, and machine learning.

Size-independent quantification of nanoplastics in various aqueous media using surfaced-enhanced Raman scattering

In this work, we successfully developed an intriguing preparation strategy to reduce the size-dependent effect of nanoplastics (NPLs), which is the limitation of NPLs quantification by surface-enhanced Raman scattering (SERS). This simple and low-cost technique enabled us to quantify different sizes (i.e., 100, 300, 600, and 800 nm) of polystyrene nanospheres (PS NSs) in various aqueous media. The SERS substrate was simply prepared by sputtering gold particles to cover on a glass cover slide. By dissolving PS NSs in toluene and preconcentrating by coffee-ring effect, SERS measurement can quantify NPLs at a very low concentration with a limit of detection (LOD) of approximately 0.10-0.26 μg/mL. The experiment was also conducted in the presence of interferences, including salts, sugars, amino acids, and detergents. The method was validated for quantitative analysis using a mixture of 100-, 300-, 600-, and 800-nm PS NSs in a ratio of 1:1:1:1 in real-world media (i.e., tap water, mineral water, and river water), which successfully approaches the evaluation of PS NSs in the range of 10-40 µg/mL with an LOD of approximately 0.32-0.52 µg/mL.

Influence of Surface Roughness on Droplet Evaporation and Absorption: Insights into Experiments from Lubrication-Theory-Based Models

While solid substrates are often idealized as being perfectly smooth, all real surfaces possess some level of topographical and chemical heterogeneity. This heterogeneity can greatly influence droplet dynamics. Mathematical models based on lubrication theory that account for surface roughness reveal how topographical defects induce contact-line pinning and affect the deposition patterns of colloidal particles suspended in the droplet. Contact-line pinning profoundly changes the behavior of droplet evaporation on horizontal and inclined impermeable substrates and droplet absorption on horizontal permeable substrates. Models accounting for surface roughness yield predictions that are qualitatively consistent with experimental observations and also provide insight into the underlying physical mechanisms. These models are a foundation for the exploration of a rich array of problems concerning droplet dynamics which are of both fundamental and practical interest.

A Rapid Therapeutic Drug Monitoring Strategy of Carbamazepine in Serum by Using Coffee-Ring Effect Assisted Surface-Enhanced Raman Spectroscopy

Carbamazepine (CBZ) has a narrow therapeutic concentration range, and therapeutic drug monitoring (TDM) is necessary for its safe and effective individualized medication. This study aims to develop a procedure for CBZ detection in serum using coffee-ring effect assisted surface-enhanced Raman spectroscopy (SERS). Silver nanoparticles deposited onto silicon wafers were used as the SERS-active material. Surface treatment optimization of the silicon wafers and the liquid-liquid extraction method were conducted to eliminate the influence of impurities on the silicon wafer surface and the protein matrix. The proposed detection procedure allows for the fast determination of CBZ in artificially spiked serum samples within a concentration range of 2.5-40 μg·mL^-1, which matches the range of the drug concentrations in the serum after oral medication. The limit of detection for CBZ was found to be 0.01 μg·mL^-1. The developed method allowed CBZ and its metabolites to be ultimately distinguished from real serum samples. The developed method is anticipated to be a potential tool for monitoring other drug concentrations.

Quartz Crystal Microbalance-Based Aptasensors for Medical Diagnosis

Aptamers are important materials for the specific determination of different disease-related biomarkers. Several methods have been enhanced to transform selected target molecule-specific aptamer bindings into measurable signals. A number of specific aptamer-based biosensors have been designed for potential applications in clinical diagnostics. Various methods in combination with a wide variety of nano-scale materials have been employed to develop aptamer-based biosensors to further increase sensitivity and detection limit for related target molecules. In this critical review, we highlight the advantages of aptamers as biorecognition elements in biosensors for target biomolecules. In recent years, it has been demonstrated that electrode material plays an important role in obtaining quick, label-free, simple, stable, and sensitive detection in biological analysis using piezoelectric devices. For this reason, we review the recent progress in growth of aptamer-based QCM biosensors for medical diagnoses, including virus, bacteria, cell, protein, and disease biomarker detection.

Controlled nucleation in evaporative crystallization using a confined-vapor driven solutal Marangoni effect

In droplet evaporation, the onset of evaporative crystallization near a contact line is inevitable if there is a coffee-ring effect increasing the local concentration of suspended particles at the edge. In this study, we present a novel idea to control the nucleation location of surfactant crystallization by using the vapor-driven solutal Marangoni effects of a binary mixture drop in a confined chamber. Here, the evaporated volatile vapors near the droplet surface can change the local surface tension and generate a radially inward flow that suppresses the conventional coffee-ring flow (i.e., evaporatively-driven capillary flow). Using this method, we could accumulate suspended particles in the middle of the droplet. In consequence, we succeed in adjusting the nucleation location from the droplet edge to the center provided that a gel-transition process is neglected, where the crystallized material has a relatively long chain length. Here, we tested different hydrocarbon chain lengths of the surfactants (i.e., CTAB > TTAB > DTAB). We expect that the proposed idea can offer great potential for controlling the nucleation in the evaporative crystallization and its final crystalline solid morphology.

A broad perspective to particle-laden fluid interfaces systems: from chemically homogeneous particles to active colloids

Particles adsorbed to fluid interfaces are ubiquitous in industry, nature or life. The wide range of properties arising from the assembly of particles at fluid interface has stimulated an intense research activity on shed light to the most fundamental physico-chemical aspects of these systems. These include the mechanisms driving the equilibration of the interfacial layers, trapping energy, specific inter-particle interactions and the response of the particle-laden interface to mechanical perturbations and flows. The understanding of the physico-chemistry of particle-laden interfaces becomes essential for taking advantage of the particle capacity to stabilize interfaces for the preparation of different dispersed systems (emulsions, foams or colloidosomes) and the fabrication of new reconfigurable interface-dominated devices. This review presents a detailed overview of the physico-chemical aspects that determine the behavior of particles trapped at fluid interfaces. This has been combined with some examples of real and potential applications of these systems in technological and industrial fields. It is expected that this information can provide a general perspective of the topic that can be exploited for researchers and technologist non-specialized in the study of particle-laden interfaces, or for experienced researcher seeking new questions to solve.

Redox Control of Particle Deposition from Drying Drops

The coffee-ring effect (CRE), which denotes the accumulation of nonvolatile compounds at the periphery of a pinned sessile drying drop, is a universal and ubiquitous yet complex phenomenon. It is crucial to better understand and control it, either to avoid its various deleterious consequences in many processes requiring homogeneous deposition or to exploit it for applications ranging from controlled particle patterning to low cost diagnostics. Here, we report for the first time the use of a reduction-oxidation (redox) stimulus to cancel the CRE or harness it, leading to a robust and tunable control of particle deposition in drying sessile drops. This is achieved by implementing redox-sensitive ferrocenyl cationic surfactants of different chain lengths in drying drops containing anionic colloids. Varying surfactant hydrophobicity, concentration, and redox state allows us not only to control the overall distribution of deposited particles, including the possibility to fully cancel the CRE, but also to modify the microscopic organization of particles inside the deposit. Notably, with all other parameters being fixed, this method allows the adjustment of the deposited particle patterns, from polycrystalline rings to uniform disks, as a function of the oxidation rate. We show that the redox control can be achieved either chemically by the addition of oxidants or electrochemically by applying a potential for additive-free and reversible actuation in a closed system. This correlation between the redox state and the particle pattern opens a perspective for both redox-programmable particle patterning and original diagnostic applications based on the visual determination of a redox state. It also contributes to clarify the role of surfactant charge and its amphiphilic character in directing particle deposition from drying suspensions.

Directional migration propensity of calf thymus DNA in a gradient of metal ions

The migration propensity and spatially modulated surface deposition of calf thymus DNA have been reported in response to gradients of different monovalent and divalent ions.

Controllable light-induced droplet evaporative crystallization

Droplet evaporative crystallization is one of the practical tools for clinical diagnosis, environmental monitoring, and pharmaceutical synthesis. Herein, we proposed a controllable and flexible light strategy to manipulate the droplet evaporative crystallization, in which the photothermal effect of a focused infrared laser actuated intense evaporation to attain the droplet evaporative crystallization. Due to the localized heating effect, not only the droplet evaporative crystallization could be promoted, but also the resultant Marangoni-flow enabled the crystals to be concentrated, exhibiting excellent controllability. Besides, a relationship between the crystallization starting time and the solution concentration/laser power was achieved, which benefited the manipulation of the droplet evaporative crystallization. The light strategy proposed in the present study possesses promising potential for future applications.

Dendritic nanoparticle self-assembly from drying a sessile nanofluid droplet

The pattern formation left by a drying nanofluid droplet is related to the evaporation induced particle self-assembly. The experimental results demonstrate the formation of dendritic particle deposition after the liquid phase of unpinned sessile nanofluid droplets is fully evaporated. The dried-in particle assemblies exhibit the dendritic patterns connecting the sprawling branches with a central core structure. The branched structures are formed by particles merging in the receding front. A three-dimensional lattice-gas kinetic Monte Carlo model is developed to simulate the particle self-assembling behaviour in a drying particle-laden droplet with the dewetting three-phase line. The parameter study is carried out to demonstrate the trend of the dendritic pattern formation. The various patterns are simulated by varying the chemical potentials and the interaction energies among particles, liquids, and substrates. The dendritic particle depositions are measured in three dimensions after the nanofluid droplet is completely dried. Qualitative agreement is observed between the experimental and the numerical results. Thicker branches and larger central cores are observed with an increase of particle concentrations.

Pattern Formation upon Evaporation of Sessile Droplets of Polyelectrolyte/Surfactant Mixtures on Silicon Wafers

The formation of coffee-ring deposits upon evaporation of sessile droplets containing mixtures of poly(diallyldimethylammonium chloride) (PDADMAC) and two different anionic surfactants were studied. This process is driven by the Marangoni stresses resulting from the formation of surface-active polyelectrolyte-surfactant complexes in solution and the salt arising from the release of counterions. The morphologies of the deposits appear to be dependent on the surfactant concentration, independent of their chemical nature, and consist of a peripheral coffee ring composed of PDADMAC and PDADMAC-surfactant complexes, and a secondary region of dendrite-like structures of pure NaCl at the interior of the residue formed at the end of the evaporation. This is compatible with a hydrodynamic flow associated with the Marangoni stress from the apex of the drop to the three-phase contact line for those cases in which the concentration of the complexes dominates the surface tension, whereas it is reversed when most of the PDADMAC and the complexes have been deposited at the rim and the bulk contains mainly salt.

Light-Fueled Beating Coffee-Ring Deposition for Droplet Evaporative Crystallization

Condensed deposition favors biochemical analysis, bioassays, and clinical diagnosis, but the existing strategies may suffer from low resolution, inaccurate control, cross-contamination, or miscellaneous apparatus. Herein, we propose a noncontact light strategy to enable the condensed deposition for droplet evaporative crystallization, in which the photothermal effect of a focused infrared laser is employed to induce intense evaporation. Due to the localized heating effect, not only can the droplet evaporative crystallization on the hydrophobic substrate be promoted, but also the resultant intensified Marangoni flow enables the movement of the early-formed crystals, preventing the pinning of the triple-phase contact line. Synergy of the Marangoni flow and nonuniform evaporation makes the solutes tend to accumulate near the focused light beam region, which facilitates the condensed deposition. More importantly, this light strategy not only enables condensed deposition on the hydrophobic surface with low hysteresis, but also works successfully on the hydrophilic substrate with high hysteresis via adjusting input laser power. It is demonstrated that the light strategy proposed in the present study has great potential for relevant applications.

Influence of arabinoxylan on the drying of cellulose nanocrystals suspension: From coffee ring to Maltese cross pattern and application to enzymatic detection

Evaporation of sessile droplet containing suspension of cellulose nanocrystals (CNC) results on birefringent coffee ring pattern (CR), due to the concentration increase and self-assembly of CNC carried by the flow at the edge of evaporating droplet. In this work, we studied the apparition of Maltese cross pattern, (MC) after addition of an hydrosoluble biopolymer belonging to the hemicellulose family, i.e. arabinoxylan (AX). To investigate the mechanisms that control MC pattern apparition, distribution of the two components inside the dried droplet was investigated using FTIR. CNC and AX were found to be homogenously deposited and CNC self-assembly induces nanoparticles orientation in the CR deposit. We demonstrate that the increase of concentration during drying induces gelation of CNC/AX mixture leading to MC pattern apparition. We take advantage of the apparition of MC pattern to develop a novel catalytic activity detection assay based on the variation of viscosity. Indeed, addition of Endo-1,4-β-Xylanase (Xyl) addition to a suspension containing CNC/AX complex leads to hydrolysis of AX that decrease in droplet viscosity leading to MC disappearance. The enzymatic detection assay is thus simple, easy to handle, fast, sensitive and do not require complex analytical devices.

Applying droplets and films in evaporative lithography

This review covers experimental results of evaporative lithography and analyzes existing mathematical models of this method. Evaporating droplets and films are used in different fields, such as cooling of heated surfaces of electronic devices, diagnostics in health care, creation of transparent conductive coatings on flexible substrates, and surface patterning. A method called evaporative lithography emerged after the connection between the coffee ring effect taking place in drying colloidal droplets and naturally occurring inhomogeneous vapor flux densities from liquid-vapor interfaces was established. Essential control of the colloidal particle deposit patterns is achieved in this method by producing ambient conditions that induce a nonuniform evaporation profile from the colloidal liquid surface. Evaporative lithography is part of a wider field known as "evaporative-induced self-assembly" (EISA). EISA involves methods based on contact line processes, methods employing particle interaction effects, and evaporative lithography. As a rule, evaporative lithography is a flexible and single-stage process with such advantages as simplicity, low price, and the possibility of application to almost any substrate without pretreatment. Since there is no mechanical impact on the template in evaporative lithography, the template integrity is preserved in the process. The method is also useful for creating materials with localized functions, such as slipperiness and self-healing. For these reasons, evaporative lithography attracts increasing attention and has a number of noticeable achievements at present. We also analyze limitations of the approach and ways of its further development.

Complex Pattern Formation in Solutions of Protein and Mixed Salts Using Dehydrating Sessile Droplets

A sessile droplet of a complex fluid exhibits several stages of drying leading to the formation of a final pattern on the substrate. We report such pattern formation in dehydrating droplets of protein (BSA) and salts (MgCl₂ and KCl) at various concentrations of the two components (protein and salts) as part of a parametric study for the understanding of complex patterns of dehydrating biofluid droplets (blood and urine), which will eventually be used for diagnosis of bladder cancer. The exact analysis of the biofluid patterns will require a rigorous parametric study; however, the current work provides an initial understanding of the effect of the basic components present in a biofluid droplet. Arrangement of the protein and the salts, due to evaporation, leads to the formation of some very distinctive final structures at the end of the droplet lifetime. Furthermore, these structures can be manipulated by varying the initial ratio of the two components in the solution. MgCl₂ forms chains of crystals beyond a threshold initial concentration of protein (>3 wt %). However, the formation of such a crystal is also limited by the maximum concentration of the salt initially present in the droplet (≤1 wt %). On the other hand, KCl forms dendritic and rectangular crystals in the presence of BSA. The formation of these crystals also depends on the relative concentration of salt and protein in the droplet. We also investigated the dried-out patterns in dehydrating droplets of mixed salts (MgCl₂ + KCl) and protein. The patterns can be tuned from a continuous dendritic structure to a snow-flake type structure just by altering the initial ratio of the two salts in the mixture, keeping all other parameters constant.

Investigating the Effect of Antibody-Antigen Reactions on the Internal Convection in a Sessile Droplet via Microparticle Image Velocimetry and DLVO Analysis

The evaporation of antigen-laden sessile droplets on antibody-immobilized PDMS substrates could be used in place of microwells for detection purposes owing to the lesser requirements of analytes and a reduced reaction time. To develop such techniques, the effects of different parameters on the reaction efficiency and on the resulting deposition patterns of antigens on the surface after evaporation need to be well understood. While the resultant deposition patterns from the evaporation of droplets of biological fluids on surfaces are being studied for various biomedical applications, systems where the analyte of interest in the droplet binds to the surface have not been investigated until now. While the effect of temperature on the internal convection within sessile droplets has been studied, the effect of the analyte (antigen in this work) concentration and the analyte-surface (antigen-antibody in this work) binding on the internal convection has not been studied until now. Therefore, to gain insight, the evaporation dynamics of sessile droplets with different concentrations of antigens along with polystyrene microspheres (used as tracers) in phosphate-buffered saline (PBS) on antibody-immobilized PDMS substrates were experimentally studied using microparticle image velocimetry (PIV). It was found that Marangoni flow due to concentration gradients and surface reactions was responsible for the observed velocity field. The antibody-antigen reaction (as compared to the control case of no surface reaction) and higher concentrations of prostate specific antigen (PSA) resulted in increased strength of Marangoni convection. To obtain further insight into the different deposition patterns obtained, the contributions of different particle-particle and particle-substrate forces were determined, and it was observed that the Marangoni forces along with surface tension and DLVO forces create a uniform deposition of the particles present within the droplet. This learning could be used to design biosensors.

Wettability read-out strategy for aptamer target binding based on a recognition/hydrophobic bilayer surface

We established a wettability read-out strategy for an aptasensor with a bilayer surface, using hydrophobicity reduction as the signal. The signal was induced by a fracture of the nanoneedle layer resulting from recognition by the supporting aptamer layer. The kinetics and influencing factors of this process were investigated with time curves.

Suppression of coffee-ring effect via periodic oscillation of substrate for ultra-sensitive enrichment towards surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) has attracted extensive interest due to excellent molecule recognition and sensitive concentration detection. Nevertheless, the coffee ring effect (CR) during the analyte evaporation always causes an uneven distribution of the assembled hot-spots, and hence the unreliable SERS signal is produced. In this study, for the first time, we present a suppressed coffee ring (SCR) system via a combination of a magnetically functionalized membrane and reciprocating magnetic field to dynamically suppress the CR for highly reliable and ultra-sensitive SERS detection. The enrichment mechanism of the nanoparticles and the analyte molecules within the sessile droplet based on the proposed system was studied. We experimentally observed that the driving frequency could well affect the final pattern, and typically a higher driving frequency facilitated a smaller coverage area with better enrichment performance. With the use of R6G molecule and (100 nm) gold nanoparticles, we examined the uniformity and SERS of the assembled 'hot-spots' in the SCR system. The results indicate that the uniformity can be greatly improved via SCR in comparison of ring stain, with the RSD of a Raman signal as low as 7.1% even at a low concentration of 10^-12 mol L^-1. Such system also enables the further enhancement in the SERS signal, with the detection limit down to 10^-16 mol L^-1, the enhancement factor magnitude up to 10¹³, and the linear relationship between the SERS intensity and the analyte concentrations within the range of 10^-6-10^-12 and 10^-12-10^-16 mol L^-1, respectively. The applicability of the SCR-based SERS detection for diverse analytes was also proved with a similar but further enhanced signal of MB and 4-ATP. We believe that the excellent SCR-based SERS performance via the proposed system has great potentials for ultra-sensitive detection and/or precise quantitative analysis in various research fields and applications.

Duplexed aptamers: history, design, theory, and application to biosensing

Nucleic acid aptamers are single stranded DNA or RNA sequences that specifically bind a cognate ligand. In addition to their widespread use as stand-alone affinity binding reagents in analytical chemistry, aptamers have been engineered into a variety of ligand-specific biosensors, termed aptasensors. One of the most common aptasensor formats is the duplexed aptamer (DA). As defined herein, DAs are aptasensors containing two nucleic acid elements coupled via Watson-Crick base pairing: (i) an aptamer sequence, which serves as a ligand-specific receptor, and (ii) an aptamer-complementary element (ACE), such as a short DNA oligonucleotide, which is designed to hybridize to the aptamer. The ACE competes with ligand binding, such that DAs generate a signal upon ligand-dependent ACE-aptamer dehybridization. DAs possess intrinsic advantages over other aptasensor designs. For example, DA biosensing designs generalize across DNA and RNA aptamers, DAs are compatible with many readout methods, and DAs are inherently tunable on the basis of nucleic acid hybridization. However, despite their utility and popularity, DAs have not been well defined in the literature, leading to confusion over the differences between DAs and other aptasensor formats. In this review, we introduce a framework for DAs based on ACEs, and use this framework to distinguish DAs from other aptasensor formats and to categorize cis- and trans-DA designs. We then explore the ligand binding dynamics and chemical properties that underpin DA systems, which fall under conformational selection and induced fit models, and which mirror classical SN1 and SN2 models of nucleophilic substitution reactions. We further review a variety of in vitro and in vivo applications of DAs in the chemical and biological sciences, including riboswitches and riboregulators. Finally, we present future directions of DAs as ligand-responsive nucleic acids. Owing to their tractability, versatility and ease of engineering, DA biosensors bear a great potential for the development of new applications and technologies in fields ranging from analytical chemistry and mechanistic modeling to medicine and synthetic biology.

Tailored 3D printed micro-crystallization chip for versatile and high-efficiency droplet evaporative crystallization

Droplet evaporative crystallization on a micro-structured platform with limited interfacial area has potential applications in crystallization theory, bioengineering, and particle drug preparation. Here, an efficient and versatile approach is discussed for multiple drop-evaporative crystallization processes on a micro-crystallization chip fabricated via three-dimensional printing. A chip with limited interfacial area could be fabricated on a highly controlled crystallizer interface. During liquid injection, various drop locations and evaporative conditions can be used, which enables flexible and distinct crystallization processes. This reveals controlling mechanisms and identifies nucleation locations and growth paths. Various classic crystallization systems were introduced to evaluate the chip performance. Controlled nucleation and growth mechanisms at stable evaporative rates were revealed. From the final crystal morphologies, particle locations, and distributions, the effects of the initial concentration and droplet contact conditions at the triple-phase interface could be investigated with high adjustability. Moreover, the results can provide insights into the 'coffee ring' formation during evaporative crystallization, dendritic crystal growth, and hydrate crystallization mechanisms. In the limited microstructure, the capillary flow of a liquid drop can spontaneously drive the crystal distribution and morphology. Finally, incorrect liquid drop locations that led to unpredictable crystal formation and distributions were discussed to improve repeatability and efficiency. Applications include the manufacture of particle drugs and flow chemistry.

Visualized Quantitation of Trace Nucleic Acids Based on the Coffee-Ring Effect on Colloid-Crystal Substrates

We report a visualized quantitative detection method for nucleic acid amplification tests based on the coffee-ring effect on colloid-crystal substrates. The solution for loop-mediated isothermal amplification (LAMP) of DNA is drop cast on a colloid-crystal surface. After complete drying, a coffee ring containing the LAMP byproduct (i.e., magnesium pyrophosphate) is formed, and it is found that the width of the coffee ring is linearly correlated to the logarithm of the original DNA concentration before the isothermal amplification. Importantly, compared with other substrates, we found that the colloid-crystal substrate is an appropriate substrate for carrying out the assay of high sensitivity. On the basis of these findings, we develop a coffee-ring-based assay for quantitative readout of trace DNA in a sample. The assay requires 0.50 μL of the sample and is completed in 5 min in a homemade chamber with constant humidity. Semiquantitative detection of trace DNA is performed using naked eyes. With the use of a smartphone, the DNA in a sample can be quantitatively detected with a limit of detection of 20 copies.

A review on suppression and utilization of the coffee-ring effect

Evaporation of sessile droplets containing non-volatile solutes dispersed in a volatile solvent leaves behind ring-like solid stains. As the volatile species evaporates, pinning of the contact line gives rise to capillary flows that transport non-volatile solutes to the contact line. This phenomenon, called the coffee-ring effect, compromises the overall performance of industrially relevant manufacturing processes involving evaporation such as printing, biochemical analysis, manufacturing of nano-structured materials through colloidal and macromolecular patterning. Various approaches have been developed to suppress this phenomenon, which is otherwise difficult to avoid. The coffee-ring effect has also been leveraged to prepare new materials through convection induced assembly. This review underlines not only the strategies developed to suppress the coffee-ring effect but also sheds light on approaches to arrive at novel processes and materials. Working principles and applicability of these strategies are discussed together with a critical comparison.

Converting colour to length based on the coffee-ring effect for quantitative immunoassays using a ruler as readout

We report a method for converting the colorimetric results of an enzyme-linked immunosorbent assay (ELISA) into length based on the coffee-ring effect, so that the quantitative detection of analytes can be carried out simply using a ruler. The influence of the shape and lamination of the paper strip on the test results is studied. As a demonstration, human IgG is quantitatively analyzed. It is found that the width of the colored stains correlates with the concentration of the analyte which can be measured for quantitative analysis. The method is promising for quantitative point-of-care detection of biomarkers under resource-limited settings.

A Low-Cost Inkjet-Printed Aptamer-Based Electrochemical Biosensor for the Selective Detection of Lysozyme

Recently, inkjet-printing has gained increased popularity in applications such as flexible electronics and disposable sensors, as well as in wearable sensors because of its multifarious advantages. This work presents a novel, low-cost immobilization technique using inkjet-printing for the development of an aptamer-based biosensor for the detection of lysozyme, an important biomarker in various disease diagnosis. The strong affinity between the carbon nanotube (CNT) and the single-stranded DNA is exploited to immobilize the aptamers onto the working electrode by printing the ink containing the dispersion of CNT-aptamer complex. The inkjet-printing method enables aptamer density control, as well as high resolution patternability. Our developed sensor shows a detection limit of 90 ng/mL with high target selectivity against other proteins. The sensor also demonstrates a shelf-life for a reasonable period. This technology has potential for applications in developing low-cost point-of-care diagnostic testing kits for home healthcare.

Drying of Droplets of Colloidal Suspensions on Rough Substrates

In many technological applications, excess solvent must be removed from liquid droplets to deposit solutes onto substrates. Often, the substrates on which the droplets rest may possess some roughness, either intended or unintended. Motivated by these observations, we present a lubrication-theory-based model to study the drying of droplets of colloidal suspensions on a substrate containing a topographical defect. The model consists of a system of one-dimensional partial differential equations accounting for the shape of the droplet and depth-averaged concentration of colloidal particles. A precursor film and disjoining pressure are used to describe the contact-line region, and evaporation is included using the well-known one-sided model. Finite-difference solutions reveal that when colloidal particles are absent, the droplet contact line can pin to a defect for a significant portion of the drying time due to a balance between capillary-pressure gradients and disjoining-pressure gradients. The time-evolution of the droplet radius and contact angle exhibits the constant-radius and constant-contact-angle stages that have been observed in prior experiments. When colloidal particles are present and the defect is absent, the model predicts that particles will be deposited near the center of the droplet in a cone-like pattern. However, when a defect is present, pinning of the contact-line accelerates droplet solidification, leading to particle deposition near the droplet edge in a coffee-ring pattern. These predictions are consistent with prior experimental observations, and illustrate the critical role contact-line pinning plays in controlling the dynamics of drying droplets.

Single-Drop Raman Imaging Exposes the Trace Contaminants in Milk

Better milk safety control can offer important means to promote public health. However, few technologies can detect different types of contaminants in milk simultaneously. In this regard, the present work proposes a single-drop Raman imaging (SDRI) strategy for semiquantitation of multiple hazardous factors in milk solutions. By developing SDRI strategy that incorporates the coffee-ring effect (a natural phenomenon often presents in a condensed circle pattern after a drop evaporated) for sample pretreatment and discrete wavelet transform for spectra processing, the method serves well to expose typical hazardous molecular species in milk products, such as melamine, sodium thiocyanate and lincomycin hydrochloride, with little sample preparation. The detection sensitivity for melamine, sodium thiocyanate, and lincomycin hydrochloride are 0.1 mg kg^-1, 1 mg kg^-1, and 0.1 mg kg^-1, respectively. Theoretically, we establish that the SDRI represents a novel and environment-friendly method that screens the milk safety efficiently, which could be well extended to inspection of other food safety.

Sessile droplets for chemical and biological assays

Sessile droplets are non-movable droplets spanning volumes in the nL-to-μL range. The sessile-droplet-based platform provides a paradigm shift from the conventional, flow-based lab-on-a-chip philosophy, yet offering similar benefits: low reagent/sample consumption, high throughput, automation, and most importantly flexibility and versatility. Moreover, the platform relies less heavily on sophisticated fabrication techniques, often sufficient with a hydrophobic substrate, and no pump is required for operation. In addition, exploiting the physical phenomena that naturally arise when a droplet evaporates, such as the coffee-ring effect or Marangoni flow, can lead to fascinating applications. In this review, we introduce the physics of droplets, and then focus on the different types of chemical and biological assays that have been implemented in sessile droplets, including analyte concentration, particle separation and sorting, cell-based assays, and nucleic acid amplification. Finally, we provide our perspectives on this unique micro-scale platform.