Single-Step Fabrication of High-Throughput Surface-Enhanced Raman Scattering Substrates

A novel array-type microdroplet parallel-generation device

In this study, the innovative design of a new array microdroplet parallel-generation device is proposed based on the principle of fluid inertial force using a capillary glass needle. The entire device used an electromagnetic actuator as the power source. It was designed as a 9-channel parallel array of glass needles. All glass needles feed independently, allowing different solutions to be sprayed simultaneously while effectively avoiding cross-contamination. We achieved non-contact parallel precision dispensing of nanoliter-sized microdroplet arrays using a relatively simple method. In this study, we first investigated the homogeneity of the generated droplet arrays and the stability of the device over long periods of operation. Then, the influence of the driving-voltage amplitude of the electromagnet and nozzle diameter on microdroplet generation was analyzed. Finally, a prediction model for the droplet size was developed using regression analysis to investigate the on-demand generation of droplets. In summary, the device designed in this study had a novel design, low cost, and modular assembly. It has excellent potential for applications in high precision and low-volume microdroplet-array generation.

pH-responsive SERS substrates based on AgNP-polyMETAC composites on patterned self-assembled monolayers

Patterned silver nanoparticle (NP)-poly[2-(methacryloyloxy)ethyl] trimethyl ammonium chloride (AgNP-polyMETAC) composites were prepared by electrochemical lithography, surface-initiated atom-transfer radical polymerization (SI-ATRP) and NP growth inside the polymer brushes. For this purpose, polymer brushes of poly[2-(methacryloyloxy)ethyl] trimethyl ammonium chloride (polyMETAC) were utilized as strong electrolyte brush system. These were introduced in form of patterned polymer brushes to create pH-responsive surface enhanced Raman scattering SERS substrates. It is well-known that the charges of strong polyelectrolyte chains are usually insensitive to pH changes, hence, rarely strong polyelectrolyte brushes have been utilized so far to study pH-responsive properties of such films. Here pH-insensitive polyMETAC brushes exhibit pH-sensitive properties and can be used as pH-responsive surfaces for SERS applications due to the embedding of AgNPs into the polymer brushes. When increasing the pH, the assembly of the AgNPs transfers from quasi two-dimensional (2D) aggregates, attaching mainly to the polymer surface, into a three-dimensional (3D) assembly, where the particles are penetrating into the brushes. These changes result in significant alterations of the SERS efficiency of the polymer brush composite. At pH 5, the enhancement of the Raman scattering approaches its maximum. The fabricated SERS substrates show a high sensitivity as well as good experimental reliability at different pH values. Moreover, electrochemical lithography was utilized to fabricate patterned SERS substrate, which allows an easy combination of multiple other functionalities in hierarchical structuring steps. In addition, the microstructure is in our studies beneficial because of a simplified and reliable characterization of the polymer brushes at defined sample areas. The introduction of the microstructured brush system is regarded moreover attractive for the development of high-throughput platforms for rapid, automated screening and analysis applications.

Programmable Liquid Adhesion on Bio-Inspired Re-Entrant Structures

Surfaces combining antispreading and high adhesion can find wide applications in the manipulation of liquid droplets, generation of micropatterns and liquid enrichment. To fabricate such surfaces, almost all the traditional methods demand multi-step processes and chemical modification. And even so, most of them cannot be applied for some liquids with extremely low surface energy. In the past decade, multiply re-entrant structures have aroused much attention because of their universal and modification-independent antiadhesion or antipenetration ability. Unfortunately, theories and applications about their liquid adhesion behavior are still rare. In this work, inspired by the springtail skin and gecko feet in the adhered state, it is demonstrated that programmable liquid adhesion is realized on the 3D-printed micro doubly re-entrant arrays. By arranging the arrays reasonably, three different Cassie adhesion behaviors can be obtained: I) no residue adhesion, II) tunable adhesion, and III) absolute adhesion. Furthermore, various arrays are designed to tune macro/micro liquid droplet manipulation, which can find applications in the transportation of liquid droplets, liquid enrichment, generation of tiny droplets, and micropatterns.