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Patterning is crucial for advancing perovskite materials into color conversion micro‐display applications, but achieving inkjet‐printed patterns with high performance and precision remains challenging. Here, novel CsPbBr3/PDMS nanoparticles as a promising candidate is proposed for achieving precise and perfect inkjet printing patterns. The as‐prepared nanoparticles ensure exceeding brightness, exceptional stability, uniform fluorescence emission, and high resolution reaching the highest performance of drop‐on‐demand inkjet printing. Specifically, the performance and stability of CsPbBr3/PDMS nanoparticles are enhanced through a two‐step optimization involving dodecyl benzene sulfonic acid (DBSA) ligand modification and polydimethylsiloxane (PDMS) in situ coating, resulting in the highest photoluminescence quantum yield (PLQY) of 92% among CsPbBr3/polymer core‐shell composites so far. The branched structure of DBSA and the PDMS shell provide steric hindrance and effectively prevent agglomeration during storage or patterning. The viscous PDMS coating inhibits the coffee ring effect, not only leading to excellent near‐unity uniform emission but also promoting the formation of smaller and more elaborate droplets, increasing the printing resolution by up to surprisingly 300%. Importantly, the impeccably displayed, high‐resolution patterns formed by versatile CsPbBr3/PDMS nanoparticles have demonstrated significant potential for high pixel density Micro‐LED displays, enabling efficient and stable color conversion. [ABSTRACT FROM AUTHOR]

The synthesis of TaSe3 ring‐shaped crystals displaying the coffee ring effect is investigated. By recrystallizing TaSe3 microcrystals dissolved in droplets of condensed Se gas, ring‐shaped crystals were successfully grown. This novel method for ring formation effectively addressed the issue of connecting the edges of the crystal. Consequently, the synthesis method has the capability to grow MX3 ring‐shaped crystals in any location where droplets can condense, can now be grown in specific locations, thus creating opportunities for advancements in electronic component development. [ABSTRACT FROM AUTHOR]

In this study, the ecologically pre-treated wool and cotton fabrics were digitally printed with eco-friendly plant-based inks of blue, red, yellow, and black colours obtained from the plant extracts of bio indigo leaves, quebracho red bark and the flame of the forest flowers and evaluated for coffee-ring effect. The plant-based inks constituted from the Quebracho red bark and the flame of the forest flowers extract both demonstrated reduced dispersion diameter (i.e. diffused) for the Plasma Surface Treated (PST) wool and cotton fabric than the untreated. These findings could be applied for digital printing on photonics to combat the Coffee-ring effect. The possible explanation for the reduced coffee ring effect is the phenomenon known as Marangoni flow. The predominant phytochemicals functioning as chromophores namely indigotin, tannin and butein enclosed in inks constituted from the plant extracts of bio indigo leaves, quebracho red bark and the flame of the forest flowers respectively are highlighted. Additionally, the synthesis of plant-based ink, the colour scheme adapted for determining the inks colours, and the technique of print process as implemented in the study are explained. Likewise, the Colour patch, Percentage (%) Reflectance, and, Colour difference graphs as acquired from Datacolor tools are demonstrated. To end, the wash fastness, rub fastness, and light fastness test results obtained on wool and cotton fabrics digitally printed with innovative plant-based inks are detailed. The Life Cycle Analysis and assessment of functional phytochemicals on the resultant printed fabrics are suggested for future work. [ABSTRACT FROM AUTHOR]

A low-cost and effective method is reported to identify water and synthetic milk adulteration of cow's milk using coffee ring patterns. The cow's milk samples were diluted with tap water (TW), distilled water (DW) and mineral water (MW) and drop cast onto glass slides to observe coffee ring patterns. The area of the ring, total particle area and average particle diameter were extracted from these patterns. For each ring, the ratio of total particle area versus total ring area was calculated. The area ratio, regardless of water adulterants, follows an exponential model with respect to average particle diameter. Unlike TW, the ratio for DW and MW adulterated milk are clustered and classified together with respect to the particle diameter. These results were independent of dilution level and are used for adulterant classification. The ring of milk adulterated using synthetic milk gave multiple concentric rings, flower-like structures, and oil globules throughout the dilution level. An Alexnet model was used to classify water and synthetic milk adulterants in authentic milk. The trained model could achieve 96.7% and 95.8% accuracy for binary and tertiary classification respectively. These results enable us to distinguish synthetic milk from pure milk and segregate DW and MW with respect to TW adulterated milk. [ABSTRACT FROM AUTHOR]

SPR biosensors have been extensively used for investigating protein–protein interactions. However, in conventional surface plasmon resonance (SPR) biosensors, detection is limited by the Brownian-motion-governed diffusion process of sample molecules in the sensor chip, which makes it challenging to detect biomolecule interactions at ultra-low concentrations. Here, we propose a highly sensitive SPR imaging biosensor which exploits the coffee ring effect (CRE) for in situ enrichment of molecules on the sensing surface. In addition, we designed a wavelength modulation system utilizing two LEDs to reduce the system cost and enhance the detection speed. Furthermore, a detection limit of 213 fM is achieved, which amounts to an approximately 365 times improvement compared to traditional SPR biosensors. With further development, we believe that this SPR imaging system with high sensitivity, less sample consumption, and faster detection speed can be readily applied to ultra-low-concentration molecular detection and interaction analysis. [ABSTRACT FROM AUTHOR]

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In this sustainability-oriented research, the properties of plant-based inks were evaluated denoting the viscosity range of 8.5 to 10 cP, the relative density of 1.06, conductivity value of 2.51 mS/cm, and the surface tension of 60 mN/m and pH of 4.9 to be most effective for inkjet printing. The changes in these properties to the one-month storage phase are detailed as determined with attenuated total reflectance – Fourier transform infrared spectroscopy, viscometer, and tensiometer. The varied colours of plant-based inks were stable to storage time except for the blue colour ink made from bio indigo herb that displayed agal-like sediments. After the storage phase, the plant-based inks exhibited anti-thixotropic viscosity except for yellow colour plant-based ink demonstrating thixotropic behaviour. High conductivity values of 18.5 and 15.6 mS/Cm were noted for blue and black colour plant-based inks, indicating their potential for constituting conducting inks; however, the conductivity values dropped to 7.5 and 9.5, respectively, after 1 month. The pH and surface tension were found steady during the storage period. The study of the life cycle analysis of plant-based inks is suggested for future work. The significance of this work in developing plant-based inks for inkjet printing of textiles lies in the convergence of sustainability and innovation. Plant-based inks can offer an eco-friendly alternative to traditional synthetic inks that are used currently, which provides a knowledge base for good practises meeting the environmentally conscious in the digital printing of the textile industry. These developed inks from this study can not only reduce the environmental impact but also promote a healthier ecosystem. [ABSTRACT FROM AUTHOR]

SPR biosensors have been extensively used for investigating protein–protein interactions. However, in conventional surface plasmon resonance (SPR) biosensors, detection is limited by the Brownian-motion-governed diffusion process of sample molecules in the sensor chip, which makes it challenging to detect biomolecule interactions at ultra-low concentrations. Here, we propose a highly sensitive SPR imaging biosensor which exploits the coffee ring effect (CRE) for in situ enrichment of molecules on the sensing surface. In addition, we designed a wavelength modulation system utilizing two LEDs to reduce the system cost and enhance the detection speed. Furthermore, a detection limit of 213 fM is achieved, which amounts to an approximately 365 times improvement compared to traditional SPR biosensors. With further development, we believe that this SPR imaging system with high sensitivity, less sample consumption, and faster detection speed can be readily applied to ultra-low-concentration molecular detection and interaction analysis.

In thin-layer chromatography coupled with surface-enhanced Raman spectroscopy (TLC-SERS), the coffee ring effect (CRE) describes the formation of a ring-shape spot (blank in the middle and darker on the edge) caused by the aggregation of silver nanoparticles (Ag NPs), alone (single CRE) or with the analytes (double CRE). In this work, the SCRE and DCRE were investigated in two anti-diabetic drugs, hydrophobic glibenclamide (GLB) and more hydrophilic metformin (MET). The SCRE occurred in GLB analysis, as opposed to the DCRE that occurred in MET. It was proven that for optimization of the TLC-SERS analytical procedure, it is necessary to distinguish the CRE patterns of analytes. Additionally, MET and GLB were analyzed with the developed TLC-SERS method and confirmed by another validated method using high-performance liquid chromatography. Four herbal products collected on the market were found to be adulterated with GLB or/and MET; among those, one product was adulterated with both MET and GLB, and two products were adulterated with GLB at a higher concentration than the usual GLB prescription dose. The TLC-SERS method provided a useful tool for the simultaneous detection of adulterated anti-diabetic herbal products, and the comparison of the SCRE and DCRE provided more evidence to predict CRE patterns in TLC-SERS. [ABSTRACT FROM AUTHOR]

Experimental and numerical experiments were carried out to study the coffee rings or coffee splats formed by droplet evaporation with micro or nano polystyrene sphere particles (Dp = 10 μm or 100 nm). Particle image velocimetry (PIV) and a high-resolution camera were used in this experiment, along with a temperature-controlled heater and a data-acquisition computer. The results showed that a nano particle could form a homogeneous coffee splat, instead of the common coffee ring formed when using micro particles. In order to account for this phenomenon, this paper developed a complex multiphase model, one which included the smooth particle hydrodynamics (SPH) fluid model coupled with the van der Waals equation of state for droplet evaporation, the rigid particle model of finite-size micro particles, and the point–particle model of the nanometer particles. The numerical simulation was operated on a GPU-based algorithm and tested by four validation cases. A GPU could calculate 533 times the speed of a single-core CPU for about 300,000 particles. The results showed that, for rigid solid particles, the forms emerged spontaneously on the wall, and their structure was mainly affected by the boundary wettability, and less affected by the fluid flow and thermal condition. When the wall temperature was low, it was easier for the particles to be deposited on the contact line. At high wall temperature, the coffee ring effect would be weakened, and the particles were more likely to be deposited in the droplet center. The hydrophilic surface produced a larger coffee ring compared to the hydrophobic surface. The experimental and numerical results proved that particle size could play a significant role during the particle deposition, which may be a possible route for producing uniform-distributed and nano-structure coatings. [ABSTRACT FROM AUTHOR]

Devising a high-throughput additive manufacturing method (HTAM) could aid in fabricating flexible electronic devices with marginal greenhouse gas (GG) emissions and net zero by-products. A roll-to-roll (R2R) gravure printing method is considered a typical platform of HTAM to fabricate carbon nanotube-based flexible electronic devices. However, the large variation in threshold voltage (Vth) in the printed carbon nanotube devices restricts device yield, thereby limiting their practical applications. In this study, the authors would like to show a way of minimizing the variation of Vth in the R2R gravure printed carbon nanotube thin-film transistors (TFTs) by understanding how the morphology of the printed dielectric layer can be controlled during ink drying. Since the formation of an uneven morphology after drying with higher edge thickness, also known as the coffee ring effect (CRE), is one of the key factors causing the variation in Vth, regulating the morphology of the dielectric ink is vital in minimizing the Vth variation of the TFTs printed via R2R gravure. By optimizing the rheological characteristics of the dielectric ink, a homogeneous morphology was attained without CRE, consequently minimizing the Vth variation from 19.6% to 6.9%. Furthermore, the printed device stability was improved by removing the CRE and avoiding the localized bias stress during device operation. Based on this work, the yield, and the stability of R2R gravure printed devices could be improved, enabling the rapid commercialization of additively manufactured flexible devices without the emission of GG or any harmful by-products.

To date high printing resolution and scalability, i.e., macroscale component dimensions and fast printing, are incompatible characteristics for additive manufacturing (AM) processes. It is hereby demonstrated that the combination of direct writing as an AM process with colloidal assembly enables the breaching of this processing barrier. By tailoring printing parameters for polystyrene (PS) microparticle‐templates, how to avoid coffee ring formation is demonstrated, thus printing uniform single lines and macroscale areas. Moreover, a novel "comb"‐strategy is introduced to print macroscale, crack‐free colloidal coatings with low viscous colloidal suspensions. The printed templates are transformed into ceramic microporous channels as well as photonic coatings via atomic layer deposition (ALD) and calcination. The obtained structures reveal promising wicking capabilities and broadband reflection in the near‐infrared, respectively. This work provides guidelines for printing low viscous colloidal suspensions and highlights the advancements that this printing process offers toward novel applications of colloidal‐based printed structures. [ABSTRACT FROM AUTHOR]

The fabrication of a coffee ring was studied in this work to improve its sensitivity in detecting trace analytes based on surface-enhanced Raman scattering (SERS). Gold nanoparticles were synthesized with diameters of ~40 nm through the sodium citrate reduction method, and rhodamine 6G (R6G) was employed as a probe to evaluate the performance of the fabricated coffee rings. The results showed that the coffee ring formed from the water-washed gold nanoparticles presented more orderly and regular morphology as well as better SERS properties than the unwashed ones. Furthermore, both the concentration and the amount of gold nanoparticles were found to affect its SERS performance. Using the optimized coffee ring as a SERS substrate, trace R6G with a concentration of 5 × 10−8 M was detected. This sensing platform could realize aflatoxin B1 (AFB1) detection down to 5 × 10−7 M and was demonstrated to function well in real-sample testing. [ABSTRACT FROM AUTHOR]

Hg2+ is a highly toxic heavy metal ion that poses serious risks to human health and the environment. Due to its tendency to accumulate, it can easily enter the human body through the food chain, making it crucial to develop detection sensors that mimic real environmental conditions. To achieve this, our study employed a surface-enhanced Raman scattering (SERS) sensor using two strategies. First, we designed a highly selective probe by optimizing the probe and reporter DNA strands to bind Hg2+ within a thymine-thymine mismatch. Second, we used the double coffee ring effect to concentrate the optimized probe DNA. These two strategies greatly enhanced the SERS signal, resulting in a sensor with exceptional sensitivity, a low detection limit of 208.71 fM, and superior selectivity for Hg2+. The practical application of the sensor was demonstrated by successfully detecting Hg2+ in drinking water, tap water, canned tuna, and tuna sashimi. Additionally, the experimental results were presented in a pizza-shaped SERS mapping image, allowing users to estimate Hg2+ concentrations through color, providing a user-friendly and intuitive method for data comprehension and analysis. Our study presents a promising approach for sensitive and reliable Hg2+ detection, with potential implications for environmental monitoring and food safety. [Display omitted] [ABSTRACT FROM AUTHOR]

The controllable assembly of plasmonic nanoparticles has developed as one of the most significant approaches for surface enhanced Raman spectroscopy (SERS) applications. This study developed a simple approach to improve a large-scale ordered assembly of gold nanorods (GNRs) by controlling the droplet evaporation mode on hydrophobic substrates. The hydrophobic substrate was efficiently produced by spin coating the silicone oil onto the glass slides and annealing them. The analyte molecule rhodamine (R6G) was employed as a surface-enhanced Raman scattering probe to demonstrate the potential effects of the synthesized arrays. This hydrophobic platform enables the concentration and delivery of analyte molecules into the surface enhanced Raman spectroscopy sensitive site while suppressing the coffee ring effect generated by the smooth contraction motion of the base contact radius of the droplet without any pinning. Thus, the limit of detection (LOD) of the R6G analyte was lowered to 10−10 M and the homogenous dispersion of surface enhanced Raman spectroscopy hotspots within the self-assembly reproducible surface enhanced Raman spectroscopy signal. This new method enables a broad range of packing patterns and mechanisms by changing the host nanoparticles in the dispersion.

Nowadays, the detection of heavy metals is urgent, but traditional methods partially fulfill detection requirements (e.g., time-consuming and low-sensitivity). Laser-induced breakdown spectroscopy (LIBS) assisted with the dried droplet method (DDM) provides impressive performance in hazardous metal detection. However, the coffee ring effect in the droplet drying process causes inhomogeneous distribution of heavy metals, thus reducing the accuracy and sensitivity of LIBS. Herein, we developed a superhydrophobic–superhydrophilic hybrid (SSH) substrate to suppress the coffee ring effect for stable and sensitive LIBS analysis to trace heavy metals. The superhydrophobic surface with a micro/nanostructure was prepared by coating polydimethylsiloxane and silicon dioxide nanoparticles sequentially. Also, the superhydrophilic pits (diameter of 1 mm and depth of 45 μm) were fabricated onto the superhydrophobic surface by laser processing. Trace heavy metals barium (Ba) and cadmium (Cd) were analyzed using LIBS to evaluate the analytical performance (stability and sensitivity) of the SSH substrate. The results of Ba and Cd (Ba: RSD = 12.4%, LoD = 0.34 mg/L; Cd: RSD = 6.4%, LoD = 0.22 mg/L) demonstrated the coffee ring effect suppression, solute enrichment, and ultrasensitive heavy metal element detection. All in all, the SSH substrate has great potential and good application prospect in LIBS analysis for heavy metals. [ABSTRACT FROM AUTHOR]

Gold nanoparticles-modified silver nanorod (AuNPs@AgNR) arrays were fabricated as surface-enhanced Raman spectroscopy (SERS) substrates. The coffee ring effect of the AuNPs@AgNR was explored as a preconcentration method for enriching the target analytes and increasing the "hot spots." Furthermore, methyl viologen (MV) as a toxic herbicide used in agricultural production was successfully determined to investigate the application of the coffee ring effect on AuNPs@AgNR arrays and density functional theory (DFT) was employed to calculate its vibrational modes of corresponding characteristic peaks. Good linearity was obtained in the range 0.10–100 mg/L, and the limit of detection (LOD) of MV was estimated to be 0.01 mg/L, which was lower than the US maximum residue limits (MRLs). This method was also applied to practical detection of MV in river water and apple peel with LODs of 0.10 mg/L and 0.05 mg/L, respectively. SERS results suggest that the coffee ring on AuNPs@AgNR arrays provides a promising way for monitoring environmental pollution and food safety caused by pesticides. [ABSTRACT FROM AUTHOR]

Inkjet printing technology has broad application prospects in the manufacture of optoelectronic devices due to its advantages of mask‐free and high material utilization. However, the coffee ring effect during printing can make the morphology non‐uniform and has not been thoroughly explored. In this work, we used a high‐precision in‐situ inkjet printing system to print low‐viscosity perovskite precursor inks on substrates to generate color‐converting films. The viscosity of the perovskite precursor solution was improved by changing the ratio of the 5‐AVABr additive, coupled with the choice of DMF with a higher boiling point as the solvent, which greatly reduced the evaporation of the solvent at the edge of the droplet, driving the formation of Marangoni flow which can result a uniform surface of droplets, culminating in a microarray of coffee‐free rings. Notably, the perovskite can be crystallized in situ during printing without additional annealing, relying on a pumping system to assist the solvent evaporation. With the additive, the photoluminescence quantum yield of the film increased to 39%. The size of each pixel in microarrays was controlled to be less than 60 μm and 400 pixels per inch (PPI) is realized. Our work paves the way for large‐area, high‐efficiency production of perovskite color conversion films. [ABSTRACT FROM AUTHOR]

Stretchable organic light-emitting diodes (OLEDs) are a key component of stretchable electronics. Inkjet printing is a potential processing method for stretchable and flexible OLEDs. However, improving the uniformity and stretchability of the emission layer (EML) prepared by inkjet printing is challenging. Here, we propose a strategy to simultaneously improve the uniformity and stretchability of inkjet-printed films by tuning the Marangoni flow and increasing the free volume. To verify our idea, Triton X (TX) with a lipophilic alkyl end and a hydrophilic hydroxyl end was added to the Super Yellow (SY)/polystyrene- block -polybutadiene- block -polystyrene (SBS) blend film. TX played two roles. (1) To inhibit the coffee ring effect. The surface tension of the solution decreased because the hydrophilic ends of TX repelled with the nonpolar solvent toluene to decrease the cohesion of toluene molecules on the surface. Thus, the surface tension at the edges was lower than in the middle due to the high evaporation rate at the edges during solvent evaporation. This resulted in the generation of the inward Marangoni flow to drive the solute toward the middle. Therefore, the coffee ring effect was inhibited, and a uniform film was formed. (2) To improve the stretchability. With TX, the glass transition temperature decreased because TX acted as a plasticizer to insert between the polymer chains due to the attraction between the lipophilic ends of TX and the alkyl side chains of SY. This provided more free volume for the polymer chains to move and orientate under strain, which is beneficial for the stretchability. Finally, we fabricated OLEDs with the inkjet-printed stretchable EML. At 100% strain, the luminance kept 70% of the initial luminance, much higher than that without the surfactant (33%).

With the advent of Internet of Things (IoTs) and wearable devices, manufacturing requirements have shifted toward miniaturization, flexibility, environmentalization, and customization. Inkjet printing, as a non-contact picoliter-level droplet printing technology, can achieve material deposition at the microscopic level, helping to achieve high resolution and high precision patterned design. Meanwhile, inkjet printing has the advantages of simple process, high printing efficiency, mask-free digital printing, and direct pattern deposition, and is gradually emerging as a promising technology to meet such new requirements. However, there is a long way to go in constructing functional materials and emerging devices due to the uncommercialized ink materials, complicated film-forming process, and geometrically/functionally mismatched interface, limiting film quality and device applications. Herein, recent developments in working mechanisms, functional ink systems, droplet ejection and flight process, droplet drying process, as well as emerging multifunctional and intelligence applications including optics, electronics, sensors, and energy storage and conversion devices is reviewed. Finally, it is also highlight some of the critical challenges and research opportunities. The review is anticipated to provide a systematic comprehension and valuable insights for inkjet printing, thereby facilitating the advancement of their emerging applications., (© 2024 Wiley‐VCH GmbH.)

In thin-layer chromatography coupled with surface-enhanced Raman spectroscopy (TLC-SERS), the coffee ring effect (CRE) describes the formation of a ring-shape spot (blank in the middle and darker on the edge) caused by the aggregation of silver nanoparticles (Ag NPs), alone (single CRE) or with the analytes (double CRE). In this work, the SCRE and DCRE were investigated in two anti-diabetic drugs, hydrophobic glibenclamide (GLB) and more hydrophilic metformin (MET). The SCRE occurred in GLB analysis, as opposed to the DCRE that occurred in MET. It was proven that for optimization of the TLC-SERS analytical procedure, it is necessary to distinguish the CRE patterns of analytes. Additionally, MET and GLB were analyzed with the developed TLC-SERS method and confirmed by another validated method using high-performance liquid chromatography. Four herbal products collected on the market were found to be adulterated with GLB or/and MET; among those, one product was adulterated with both MET and GLB, and two products were adulterated with GLB at a higher concentration than the usual GLB prescription dose. The TLC-SERS method provided a useful tool for the simultaneous detection of adulterated anti-diabetic herbal products, and the comparison of the SCRE and DCRE provided more evidence to predict CRE patterns in TLC-SERS.

In order to rationally apply potassium fertilizer, it is very important to realize the rapid and accurate evaluation of soil available potassium (K). Conventional methods are time-consuming, consumables-consuming and laborious. A high-efficiency method was proposed in this study to meet the demand for rapid evaluation, including rapid extraction, uniform evaporation and LIBS detection. To shorten the extraction time, we increased the oscillation frequency and removed the operation of dry filtration. Compared with the conventional extraction method of the Chinese national standard (CNS), the extraction time was reduced from 30 min to 2 min. In addition, we developed a uniform evaporation method for liquid–solid transformation on the batch-detection fixed area aluminum substrate. This method reduced the moisture interference. At the same time, increasing the liquid viscosity and restricting the liquid area and shape could reduce the coffee ring effect (CRE). The determination coefficient of the calibration curve by our method was 0.99, and the limit of quantitation reached 0.8 mg/kg. Real soil samples were taken as validation, and the average relative error between our method and the CNS method was 3.58%. The results indicate that our method combined with LIBS technology could provide a fast and accurate evaluation of soil available K. [ABSTRACT FROM AUTHOR]

Inkjet printing of graphene-based materials represents a highly promising deposition method, as benefits of its flexibility for graphical printing and simple operation process. However, the huge challenge associated with inkjet printing of graphene-based inks is to overcome the coffee ring effect, which results in ununiformity of the printed pattern. In this paper, we innovatively demonstrate a novel and facile pre-deposition assisted strategy for inkjet printing of graphene ink. By simply pre-depositing an ethanol layer, a series of procedures including homogenization, solvent exchange, post-stretching, and air drying would take place, leading to uniform deposition of graphene nanosheets with a denser structure. The as-printed pattern was proved to have a flatter surface. Subsequently, the approach was applied to construct an interdigital capacitance pressure sensor with a polyethylene terephthalate (PET) substrate. Such sensors showed high sensitivity (the relative capacitance change reached 33 under 10000 Pa), low detection limits (clearly perceive the weight of 0.1 g), and excellent stability. Notably, it exhibited negligible thermal hysteresis and decreased performance compared with the sensor fabricated by direct printing, which stems from its compact structure and lower air content. The method we proposed opens up opportunities for practical applications of inkjet printing low-cost graphene-based electronic devices with desirable performance. [Display omitted] • Proposing a novel strategy for inkjet printing graphene-based pressure sensor that can alleviate the coffee ring effect. • Uniform surface reduces the randomness of the topography, making the properties of the printing devices more controllable. • Densifying the printed lines, thus improving the conductivity of the electrodes. • Facile operation and environmentally-friendly processes make the strategy more practical. • Enhancing the sensitivity and stability, decreasing the thermal hysteresis of the as-printed graphene-based sensors. [ABSTRACT FROM AUTHOR]

• The stability of LIBS detection of Sr in liquid is improved by using a foam titanium substrate. • The porous foam titanium substrate can suppress the influence of the CRE. • The RSD of the spectral intensity of Sr element reaches a minimum of 3.7%. In order to improve the stability of Laser-Induced Breakdown Spectroscopy (LIBS) detection of dried liquid droplets, a method using foam metal substrates to suppress the coffee ring effect (CRE) is proposed. Foam titanium is employed as the sample substrate, with irregular micropores and grooves on its surface. Due to the rapid infiltration of liquid droplets into the porous substrate and the diffusion of residual droplets on the foam titanium substrate, the capillary effect between the grooves and residual droplets effectively suppresses CRE. Analysis using the Washburn theory is conducted to explain the formation of CRE on titanium alloy substrates and the suppression of the coffee ring by foam titanium substrates. Two-dimensional line scans of rectangular dried liquid droplet samples on foam titanium, titanium alloy, filter paper, and polymethyl methacrylate (PMMA) substrates are performed using LIBS to verify the formation or suppression of CRE. The experiment analyzes the influence of foam titanium substrate pore size and thickness on detection stability. The results show that the relative standard deviation (RSD) of the Sr I 407.67 nm spectral intensity in foam titanium substrates with a thickness of 1.5 mm and pore size of 50 μm is the smallest (RSD = 3.7 %), which is lower than that of titanium alloy, filter paper, and PMMA substrates (RSD = 17 %, RSD = 6.7 %, RSD = 23 %, respectively), indicating that foam titanium substrates can effectively suppress the coffee ring effect. Four calibration curves for Sr I 407.67 nm on different substrates are established, and the fitting effect of foam titanium substrates is better, with a fitting coefficient R2 = 0.997 and ARSD = 5.3 %, and a limit of detection (LOD) of 0.48 μg/mL. The results indicate that foam titanium substrates can effectively suppress the coffee ring effect, improve the detection sensitivity and prediction accuracy of heavy metals, and provide a favorable reference for platform selection in the pretreatment process of LIBS technology. [ABSTRACT FROM AUTHOR]

Experimental and numerical experiments were carried out to study the coffee rings or coffee splats formed by droplet evaporation with micro or nano polystyrene sphere particles (Dp = 10 μm or 100 nm). Particle image velocimetry (PIV) and a high-resolution camera were used in this experiment, along with a temperature-controlled heater and a data-acquisition computer. The results showed that a nano particle could form a homogeneous coffee splat, instead of the common coffee ring formed when using micro particles. In order to account for this phenomenon, this paper developed a complex multiphase model, one which included the smooth particle hydrodynamics (SPH) fluid model coupled with the van der Waals equation of state for droplet evaporation, the rigid particle model of finite-size micro particles, and the point–particle model of the nanometer particles. The numerical simulation was operated on a GPU-based algorithm and tested by four validation cases. A GPU could calculate 533 times the speed of a single-core CPU for about 300,000 particles. The results showed that, for rigid solid particles, the forms emerged spontaneously on the wall, and their structure was mainly affected by the boundary wettability, and less affected by the fluid flow and thermal condition. When the wall temperature was low, it was easier for the particles to be deposited on the contact line. At high wall temperature, the coffee ring effect would be weakened, and the particles were more likely to be deposited in the droplet center. The hydrophilic surface produced a larger coffee ring compared to the hydrophobic surface. The experimental and numerical results proved that particle size could play a significant role during the particle deposition, which may be a possible route for producing uniform-distributed and nano-structure coatings.

The sensitive and stable detection of trace heavy metals in liquid is crucial given its profound impact on various aspects of human life. Currently, nanoparticle-enhanced laser-induced breakdown spectroscopy (NELIBS) with dried droplet method (DDM) is widely applied for heavy metals detection. Nevertheless, the coffee ring effect (CRE) in DDM affects the stability, accuracy, and sensitivity of NELIBS. Here, we developed a slippery surface-aggregated substrate (SS substrate) to suppress the CRE and enrich analytes, and form a plasmonic platform for NELIBS detection. The SS substrate was prepared by infiltrating perfluorinated lubricant into the pores of PTFE membrane. The droplet, with targeted elements and gold nanoparticles, was dried on the SS substate to form the plasmonic platform for NELIBS analysis. Then, trace heavy metal elements copper (Cu) and manganese (Mn) were analyzed by NELIBS. The results of Cu (RSD = 5.60%, LoD = 3.72 μg/L) and Mn (RSD = 7.42%, LoD = 6.37 μg/L), illustrated the CRE suppression and analytes enrichment by the SS substrate. The results verified the realization of stable, accurate and sensitive NELIBS detection. And the LoDs succeeded to reach the standard limit of China (GB/T 14848-2017). Furthermore, the results for groundwater detection (relative error: 5.92% (Cu) and 4.74% (Mn)), comparing NELIBS and inductively coupled plasma mass spectrometry (ICP-MS), validated the feasibility of the SS substrate in practical applications. In summary, the SS substrate exhibits immense potential for practical application such as water quality detection and supervision. [Display omitted] • A slippery surface-aggregated substrate in dried droplet method (DDM). • Suppression of coffee ring effect and enrichment of analytes by slippery surface-aggregated substrate. • Good performance in the detection of practical groundwater with stable, accuracy, and ultrasensitive analysis using NELIBS. [ABSTRACT FROM AUTHOR]

Hydrophilic surface-enhanced Raman spectroscopy (SERS) substrates were prepared by a combination of TiO2-coatings of aluminium plates through a direct titanium tetraisopropoxide (TTIP) coating and drop coated by synthesised gold nanoparticles (AuNPs). Differences between the wettability of the untreated substrates, the slowly dried Ti(OH)4 substrates and calcinated as well as plasma treated TiO2 substrates were analysed by water contact angle (WCA) measurements. The hydrophilic behaviour of the developed substrates helped to improve the distribution of the AuNPs, which reflects in overall higher lateral SERS enhancement. Surface enhancement of the substrates was tested with target molecule rhodamine 6G (R6G) and a fibre-coupled 638 nm Raman spectrometer. Additionally, the morphology of the substrates was characterised using scanning electron microscopy (SEM) and Raman microscopy. The studies showed a reduced influence of the coffee ring effect on the particle distribution, resulting in a more broadly distributed edge region, which increased the spatial reproducibility of the measured SERS signal in the surface-enhanced Raman mapping measurements on mm scale. [ABSTRACT FROM AUTHOR]

Printed micro‐supercapacitor exhibits its flexibility in geometry design and integration, showing unprecedented potential in powering the internet of things and portable devices. However, the printing process brings undesired processing defects (e.g., coffee ring effect), resulting in severe self‐discharge of the printed micro‐supercapacitors. The impact of such problems on device performance is poorly understood, limiting further development of micro‐supercapacitors. Herein, by analyzing the self‐discharge behavior of fully printed micro‐supercapacitors, the severe self‐discharge problem is accelerated by the ohmic leakage caused by the coffee ring effect on an ultrathin polymer electrolyte. Based on this understanding, the coffee ring effect was successfully eradicated by introducing graphene oxide in the polymer electrolyte, achieving a decline of 99% in the self‐discharge rate. Moreover, the micro‐supercapacitors with uniformly printed polymer electrolyte present 7.64 F cm‐3 volumetric capacitance (14.37 mF cm‐2 areal capacitance), exhibiting about 50% increase compared to the one without graphene oxide addition. This work provides a new insight to understand the relationship between processing defects and device performance, which will help improve the performance and promote the application of printed micro‐supercapacitors. [ABSTRACT FROM AUTHOR]

The synthesis of TaSe 3 ring-shaped crystals displaying the coffee ring effect is investigated. By recrystallizing TaSe 3 microcrystals dissolved in droplets of condensed Se gas, ring-shaped crystals were successfully grown. This novel method for ring formation effectively addressed the issue of connecting the edges of the crystal. Consequently, the synthesis method has the capability to grow MX 3 ring-shaped crystals in any location where droplets can condense, can now be grown in specific locations, thus creating opportunities for advancements in electronic component development.

The fabrication of a coffee ring was studied in this work to improve its sensitivity in detecting trace analytes based on surface-enhanced Raman scattering (SERS). Gold nanoparticles were synthesized with diameters of ~40 nm through the sodium citrate reduction method, and rhodamine 6G (R6G) was employed as a probe to evaluate the performance of the fabricated coffee rings. The results showed that the coffee ring formed from the water-washed gold nanoparticles presented more orderly and regular morphology as well as better SERS properties than the unwashed ones. Furthermore, both the concentration and the amount of gold nanoparticles were found to affect its SERS performance. Using the optimized coffee ring as a SERS substrate, trace R6G with a concentration of 5 × 10−8 M was detected. This sensing platform could realize aflatoxin B1 (AFB1) detection down to 5 × 10−7 M and was demonstrated to function well in real-sample testing.

Nanomaterials have been explored as alternative matrices in MALDI-MS to overcome some of the limitations of conventional matrices. Recently, we demonstrated a new means by which nanomaterials can improve peptide ionization and detection in MALDI-MS analyses by exploiting the tendency of nanomaterials to form “coffee rings” upon drying from liquids. In the current work, we investigate how nanomaterial size and composition affect the signal enhancement of peptides through the coffee-ring effect. From studies of eight different types of nanomaterials ranging in size and composition, we find that most nanomaterials can provide signal enhancement ranging from 2- to 10-fold for individual peptides, as long as a coffee ring is formed. However, when a mixture of peptides is present in a sample, the signal enhancement is the greatest for peptides whose net charge is complementary to the nanomaterial’s surface charge. These results suggest that careful design of NM surface properties could allow for selective, enhanced MALDI-MS detection of specific peptides in complex mixtures. [ABSTRACT FROM AUTHOR]

To enhance the consistency of Laser-Induced Breakdown Spectroscopy (LIBS) in detecting dried liquid droplets, a rectangular groove substrate prepared by laser etching of a polymethyl methacrylate (PMMA) acrylic board is proposed in this work to suppress the Coffee Ring Effect (CRE). Located at the corner of the substrate, the right-angled heat source structure induces a significant inward thermal capillary flow within the groove, thus inhibiting solute capillary flow. At the later stages of heating, the edge deposition that occurs during evaporation is suppressed by the sawtooth-like microscopic grooves, along with the capillary effects in associated with the liquid. The rationale of CRE suppression by the rectangular groove substrate is theoredically analyzed based on the coupled thermal-solute capillary flow theory. With LIBS used to scan the samples within rectangular and circular groove substrates, it is revealed that the relative standard deviation (RSD) of spectral intensities for Cd I 259.28 nm and Cu I 324.27 nm in the rectangular groove substrate with a depth of 0.1 mm (RSD = 4.2%, RSD = 2.1%) is less significant than in the circular groove substrate (RSD = 41.2%, RSD = 39.3%). On average, the RSD is 7.6% and 7.3%, respectively, which indicates the effect of the rectangular groove substrate on CRE suppression. On this basis, calibration curves are drawn for Cd and Cu to indicate the good fitting results for the rectangular groove substrate, with the determination coefficient (R2) reaching 0.997 and 0.996 for Cd and Cu, respectively. The limit of detection (LOD) is 0.48 μg/mL and 0.35 μg/mL, respectively. The cross-validation root mean square error (RMSECV) is 2.02 μg/mL and 1.36 μg/mL, respectively. These results demonstrate that the rectangular groove substrate is effective in suppressing the CRE, and in enhancing the sensitivity of detection and the accuracy of heavy metal prediction, which provides guidance for platform selection in the pre-processing stage of LIBS technology. [Display omitted] • The rectangular groove substrate suppresses the coffee ring effect. • Laser-induced preparation of acrylic acid substrates controls capillary flow. • The RSDs reached at 7.6% and 7.3% for Cd and Cu. [ABSTRACT FROM AUTHOR]

In order to rationally apply potassium fertilizer, it is very important to realize the rapid and accurate evaluation of soil available potassium (K). Conventional methods are time-consuming, consumables-consuming and laborious. A high-efficiency method was proposed in this study to meet the demand for rapid evaluation, including rapid extraction, uniform evaporation and LIBS detection. To shorten the extraction time, we increased the oscillation frequency and removed the operation of dry filtration. Compared with the conventional extraction method of the Chinese national standard (CNS), the extraction time was reduced from 30 min to 2 min. In addition, we developed a uniform evaporation method for liquid–solid transformation on the batch-detection fixed area aluminum substrate. This method reduced the moisture interference. At the same time, increasing the liquid viscosity and restricting the liquid area and shape could reduce the coffee ring effect (CRE). The determination coefficient of the calibration curve by our method was 0.99, and the limit of quantitation reached 0.8 mg/kg. Real soil samples were taken as validation, and the average relative error between our method and the CNS method was 3.58%. The results indicate that our method combined with LIBS technology could provide a fast and accurate evaluation of soil available K.

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. [ABSTRACT FROM AUTHOR]

We report experimental evidence of circulation reversal in evaporating drops. The internal flow patterns and particle distribution inside the evaporating water droplet with suspended polystyrene particles was studied using the Particle Image Velocimetry (PIV) technique. Droplet placed on externally heated substrate showed two symmetric and counter rotating Marangoni convection cells at the beginning of evaporation process. Towards the final stages of evaporation, the two counter rotating Marangoni convection cells reversed the direction of circulation. Interestingly, it was observed that between these two stages four symmetric and counter rotating convection cells appeared inside the evaporating droplet. Our results are in support of recent theoretical investigations on circulation reversal in evaporating drops. The flow patterns were driven by the surface tension gradient caused by the surface temperature distribution along the droplet-air interface. The surface temperature distribution is not only dependent on the non-uniform evaporation flux but also on the conduction path from the substrate to the droplet surface. Towards the end of evaporation, the fluid flow becomes radially outward from the center to the pinned edge of the droplet. We have also investigated the evaporating droplets placed on a curved substrate. Unlike the case of flat substrate, the evaporation from curved surface resulted monotonic temperature profile. The particle distribution and resulting deposition patterns was strongly influenced by the internal fluid flow pattern and was observed to be different from that of a flat substrate. [ABSTRACT FROM AUTHOR]

The simple, easy and rapid technique of “drop casting” is widely used to prepare the surface of chemically modified electrodes in which the modifying layer is composed of particles such as nanotubes or nanoparticles and used for electro-catalysis notably for chemical sensing as well as materials evaluation. However, meaningful voltammetry requires the formation of uniformly modified surfaces for which the coffee ring and its related effects, which are overviewed, present a significant limitation on the reproducibility of the drop casted surfaces. Approaches for amelioration of the coffee ring effect by various methods are discussed.

Hydrophilic surface-enhanced Raman spectroscopy (SERS) substrates were prepared by a combination of TiO2-coatings of aluminium plates through a direct titanium tetraisopropoxide (TTIP) coating and drop coated by synthesised gold nanoparticles (AuNPs). Differences between the wettability of the untreated substrates, the slowly dried Ti(OH)4 substrates and calcinated as well as plasma treated TiO2 substrates were analysed by water contact angle (WCA) measurements. The hydrophilic behaviour of the developed substrates helped to improve the distribution of the AuNPs, which reflects in overall higher lateral SERS enhancement. Surface enhancement of the substrates was tested with target molecule rhodamine 6G (R6G) and a fibre-coupled 638 nm Raman spectrometer. Additionally, the morphology of the substrates was characterised using scanning electron microscopy (SEM) and Raman microscopy. The studies showed a reduced influence of the coffee ring effect on the particle distribution, resulting in a more broadly distributed edge region, which increased the spatial reproducibility of the measured SERS signal in the surface-enhanced Raman mapping measurements on mm scale.

A novel nanoporous analytical platform is reported to improve the stability of the dried droplet method (DDM). This nanoporous platform was made of tin dioxide (Np SnO2) substrate by electrochemical anodization from tin (Sn) slide. The DDM is a widely used sample pretreatment in analytical chemistry that involves placing a droplet of solution onto the substrate and drying for analytical testing. However, during the droplet drying process, the solutes would converge at the droplet edge and cause inhomogeneous solutes distribution. This is the coffee ring effect (CRE). The Np SnO2 has irregular nanopores, which allows droplet solutions to penetrate into the substrate rather than spreading out, effectively suppressing CRE. Theoretical models were built to explain the formation of CRE on blank tin (Sn) substrate and suppression of CRE on Np SnO2. Better results were obtained in detecting lithium (Li) using the Np SnO2 by laser-induced breakdown spectroscopy (LIBS). The line scanning results indicated that the Li emission line (670.8 nm) intensities on Np SnO2 substrate had lower relative standard deviation (RSD = 3.3%) than those on Sn substrate (RSD = 31.5%), which illustrate suppression of CRE and stability improvement on Np SnO2 substrate. Furthermore, Li calibration curves were built for LIBS with DDM. The curve using Np SnO2 substrate had better linearity (R2 = 0.997), higher precision (RSD = 4.2%), and higher sensitivity (LOD = 0.13 mg/L) than that by Sn substrate (R2 = 0.954, RSD = 17%, and LOD = 1.21 mg/L). All in all, the anodic Np SnO2 substrate can suppress CRE in DDM and hence improve the stability and precision of subsequent analysis. [ABSTRACT FROM AUTHOR]

The morphology of electrodes modified by drop casting layers of multi‐walled carbon nanotubes (MWCNTs) and platinum nanoparticles (PtNPs) is investigated and shown to exhibit significant heterogeneity, both in the form of 'patchy' surfaces and of 'coffee rings' formed after evaporation of the carrier solvent. The variation of the heterogeneity with total average coverage is studied and the consequences for quantitative analytical voltammetry assessed. Specifically, the oxidation of bromide in aqueous solution is examined in the case of MWCNTs‐modified electrodes and the oxidation of aqueous As(III) in that of PtNPs‐modified electrodes. In both cases, significant deviations from the expected voltammetry are observed, even for average coverages that correspond to much in excess of a monolayer of CNTs or NPs. The observed voltammetry is related to the observed structure and the use of such modified electrodes, without concomitant microscopic surface characterization, is caveated. [ABSTRACT FROM AUTHOR]

Self-pinning induced by the aggregation of particles at the edge of a pinned drop is a pre-requisite for the coffee ring formation. The edge (three-phase contact line) of a suspension drop on a hydrophobic surface would depin and shrink in the early stage of evaporation process. It is plausible to conjecture that the self-pinning of silica suspension drops depends on the particle size and surface property. Two substrate materials, the alkylsilane coated surfaces and the polydimethylsiloxane surfaces, and three different sizes of silica particles are used to explore the criterion of self-pinning of silica suspension drops on these hydrophobic surfaces. The evaporation process of droplets is recorded and further analyzed. The pinning concentration of silica suspensions of a fixed size linearly depends on the receding contact angle of the surface, irrelevant to the substrate material and initial particle concentration. The pinning concentration decreases along with an increase in particle size. In addition, the pinning concentrations of bi-dispersed silica (e.g. 400 + 1000 nm) suspensions have an excellent agreement with that of larger size (1000 nm) particle system. That implies that the larger particle dominates the system of bi-dispersed silica suspensions to initiate the self-pinning, further verified by SEM images. [ABSTRACT FROM AUTHOR]