Low-adhesive superhydrophobic surface-enhanced Raman spectroscopy substrate fabricated by femtosecond laser ablation for ultratrace molecular detection

Preparation of Stainless Steel Superhydrophobic Surface and Analysis of Hydrophobic Mechanism

By analyzing the application conditions of hydrothermal oxidation equipment, we found that the corrosion resistance of stainless steel is crucial. It is necessary to prepare superhydrophobic surface to improve the corrosion resistance and find a cost-effective and environmentally friendly preparation method. Therefore, this paper proposes a method combining nanosecond laser and post-treatment, which uses nanosecond laser to etch microstructure and reduces the surface energy through diverse post-treatment methods to achieve hydrophobicity. The surface morphology characteristics were studied, the wettability of various post-treatment methods was compared, and the hydrophobic mechanism was analyzed. The results show that the groove width has a significant impact on the surface morphology. Superhydrophobic surface can be obtained immediately after heat treatment and fluorosilane modification, while natural storage requires more than one month. All of the post-treatment can obtain hydrophobicity by reducing the surface energy, but the chemical composition is distinct. The cost-effective composite process of laser and heat treatment plays a guiding role in future research on the preparation of stainless steel superhydrophobic surface and has broad prospects in the future in large-scale production and application.

Transferable G/Au Film for Constructing a Variety of SERS Substrates

Surface-enhanced Raman scattering (SERS), as one of the most powerful analytical methods, undertakes important inspection tasks in various fields. Generally, the performance of an SERS-active substrate relies heavily on its structure, which makes it difficult to integrate multiple-functional detectability on the same substrate. To address this problem, here we designed and constructed a film of graphene/Au nanoparticles (G/Au film) through a simple method, which can be conveniently transferred to different substrates to form various composite SERS substrates subsequently. By means of the combination of the electromagnetic enhancement mechanism (EM) and the chemical enhancement mechanism (CM) of this structure, the film realized good SERS performance experimentally, with the enhancement factor (EF) approaching ca. 1.40 × 105. In addition, the G/Au film had high mechanical strength and had large specific surface area and good biocompatibility that is beneficial for Raman detection. By further transferring the film to an Ag/Si composite substrate and PDMS flexible film, it showed enhanced sensitivity and in situ detectability, respectively, indicating high compatibility and promising prospect in Raman detection.

Enhancement of Metal Nanostructure Deposition on Silicon Laser-Induced Periodic Surface Structures by Galvanic Replacement

We report a chemically motivated, single-step method to enhance metal deposition onto silicon laser-induced periodic surface structures (LIPSSs) using reactive laser ablation in liquid (RLAL). Galvanic replacement (GR) reactions were used in conjunction with RLAL (GR-RLAL) to promote the deposition of Au and Cu nanostructures onto a Si LIPSS. To increase the deposition of Au, sacrificial metals Cu, Fe, and Zn were used; Fe and Zn also enhanced the deposition of Cu. We show that the deposited metal content, surface morphology, and metal crystallite size can be tuned based on the difference in electrochemical potentials of the deposited and sacrificial metal. Compared to the Au and Cu reference samples, GR more than doubled the metal content on the LIPSS and reduced metal crystallite sizes by up to 20%. The ability to tune the metal content and crystalline domain size simultaneously makes GR-RLAL a potentially useful approach in the manufacturing of functional metal-LIPSS materials such as surface-enhanced Raman spectroscopy substrates.

Rational design of wettability-patterned microchips for high-performance attomolar surface-enhanced Raman detection

Recent progress in wettability-patterned microchips has facilitated the development of ultra-trace detection in multiple biomedical and food safety fields. The existence of a superhydrophilic trap can realize targeted deposition of the analyte. However, the wetting transition from the Cassie-Baxter state to the Wenzel state usually occurs during evaporation and leads to a larger deposition footprint, which has a strong impact on the detection sensitivity and uniformity. In this paper, we report an integrated design, fabrication, and evaporation strategy to avoid the transition for high-performance attomolar surface-enhanced Raman scattering (SERS) detection. An improved force balance model was proposed to design the microstructures of wettability-patterned microchips, which were fabricated by nanosecond laser direct writing and surface fluorination. The microchips were composed of superhydrophobic micro-grooves and superhydrophilic traps, by which the targeted deposition of Au nanoparticles and rhodamine 6G (R6G) onto a minimal area of ∼70 × 70 μm² was realized after a two-step heated evaporation. Accordingly, the detection limit was down to the attomolar level (5 × 10^-18 M) with SERS enhancement factors (EFs) exceeding 10¹⁰. More importantly, the Raman signals showed good uniformity (RSD of 11.5%) for the concentration of 2 × 10^-17 M. A good linear relationship was obtained in the quantitative concentration range of 10^-12 M to 5 × 10^-18 M with a high correlation coefficient (R2) of 0.996. These wettability-patterned microchips exhibit high performance (that is, both good sensitivity and good uniformity) in the detection of ultra-trace molecules in aqueous solutions, avoiding the need for expensive equipment and considerable skill in operations. The proposed strategy could also be applied to other microfluidic devices for rapid and simple analyte pre-concentration.

Hydrophobic plasmonic silver membrane as SERS-active catcher for rapid and ultrasensitive Cu(II) detection

The rapid and selective identification of heavy metal ions is crucial for environmental water safety. In this study, a novel surface-enhanced Raman scattering (SERS)-active catcher was designed for Cu(II) detection using a hydrophobic hydroxyoxime-mediated plasmonic silver membrane (HOX@Ag-PVDF). Uniformly dispersed Ag nanoparticles (ca. 80 nm) and hydroxyoxime molecules were synchronously decorated on the skeleton of the polyvinylidene fluoride membrane via an in situ interfacial assembly strategy. HOX@Ag-PVDF shows excellent SERS activity (EF = 2.5 × 10⁷), high reproducibility (~8% RSD), and long-term stability (50 days) for detecting 4-nitrothiophenol (4-NTP). Moreover, HOX@Ag-PVDF can serve as a new platform for rapid and dry-free SERS detection of Cu(II) owing to its strong affinity and surface hydrophobicity. Cu(II) ions can be rapidly captured in 5 s and selectively recognized by SERS signals without interference from other metal ions. HOX@Ag-PVDF exhibits linear SERS response signals at low concentrations ranging from 10^-6 to 10^-10 mol/L Cu(II) (R² = 0.9893) with a low detection limit (LOD) of 52.0 pmol/L. This hydrophobic plasmonic membrane, with its simple sampling and rapid SERS response characteristics, provides ultrasensitive recognition and heavy metal detection for practical applications.

Extremely Sensitive SERS Sensors Based on a Femtosecond Laser-Fabricated Superhydrophobic/-philic Microporous Platform

The detection of molecules from highly diluted solutions with a limited amount is vital for precancer diagnosis, food safety, and forensic analysis. The sensitivity and convenience of detection techniques are the primary concerns. In this study, a hybrid superhydrophobic/-philic (SH/SHL) microporous platform is designed and fabricated by a femtosecond laser to improve surface-enhanced Raman scattering (SERS) performances. Relying on the micropores fabricated at the center of SHL patterns, sediments distributed at the central regions are avoided, leading to the further enrichment of the target molecules. The engineered micropores with high identification further improve the speed of Raman tests, and the fabricated SERS substrate shows an advantage in outdoor handheld detection and automated inspection applications. The optimized SERS sensor is sufficient for attomolar-level detection (10^-17 M) of rhodamine 6G using analyte volumes of just 5 μL, corresponding to an enhancement factor of 5.19 × 10¹³. Meanwhile, a relative standard deviation of 7.48% at 10^-10 M shows the excellent uniformity of this proposed SERS platform. This work further pushes forward the practical applications of SERS technology in ultratrace molecular detections.

Superhydrophilic-Superhydrophobic Multifunctional Janus Foam Fabrication Using a Spatially Shaped Femtosecond Laser for Fog Collection and Detection

Fog collection is an effective method for addressing water shortages in arid areas. By constructing a Janus structure with asymmetric wettability on its two sides, flexible and efficient fog capture can be achieved. However, in situ detection and fog collection on a Janus surface are still challenging tasks. Herein, a novel method for producing a superhydrophilic-superhydrophobic Janus fog collector is proposed; the method utilizes a combined process in which a spatially shaped femtosecond laser treatment (superhydrophilic) is applied to one side of a copper foam and a chemical replacement reaction (superhydrophobic) is applied to the other side of the copper foam. Two configurations of the Janus structure were designed to study different water transport behaviors. Furthermore, the Au micro-nanoparticle prepared adhered to the Janus structure, indicating the effectiveness of surface-enhanced Raman spectroscopy detection. The Janus foam shows excellent sensitivity and stability on testing the fog mixed with rhodamine 6G. This surface allows for the simultaneous collection and detection of fog, which can provide insights into the preparation of Janus multifunction structures and how such structures can play a key role in the subsequent purification and usage of water resources.

SERS substrate with wettability difference for molecular self-concentrating detection

The surface-enhanced Raman spectroscopy (SERS) has attracted much attention due to the powerful capability of quantificational analysis. Nowadays, most of the enhancement effect by SERS substrate is provided by the 'hot spots' occupying relatively small space. When the amount of analyte is too low, it is difficult to ensure that all the probe molecules can be placed into the 'hot spots', which is a headache in SERS quatification. In order to solve this problem, we have developed a structure of CuO nanowires/Ag nanoparticles with wettability capacity difference, which can aggregate molecules in water and oil simultaneously under two different mechanisms. The limit of detection and enhancement factor of this structure are estimated as 10^-15M and 1.55 × 10¹¹respectively (for rhodamine 6G, R6G). In a proof-in-principle experiment of sewage detection, it successfully achieved the aggregation and additional enhancement of both the R6G molecules in aqueous solution and thiuram molecules in toluene, realizing efficient and accurate Raman detection.

Femtosecond laser patterned superhydrophobic/hydrophobic SERS sensors for rapid positioning ultratrace detection

Ultratrace molecular detections are vital for precancer diagnosis, forensic analysis, and food safety. Superhydrophobic (SH) surface-enhanced Raman scattering (SERS) sensors are regarded as an ideal approach to improve detection performance by concentrating analyte molecules within a small volume. However, due to the low adhesion of SH surfaces, the analyte droplet is prone to rolling, making it hard to deposit molecules on a predetermined position. Furthermore, the sediment with a very small area on the SH-SERS surface is difficult to be captured even with a Raman microscope. In this study, femtosecond laser fabricated hybrid SH/hydrophobic (SH/HB) surfaces are successfully applied to realize a rapid and highly sensitive SERS detection. By modulating dual surface structures and wetting behaviors, the analyte molecules can be enriched at the edge of HB pattern. This improves the convenience and speed of Raman test. On a hybrid SH/HB SERS substrate with a circular HB pattern at 300-µm-diameter, a femtomolar level (10^-14 M) of rhodamine 6G can be detected by using analyte volumes of just 5 µL. The SERS enhancement factor can reach 5.7×10⁸ and a good uniformity with a relative standard deviation of 6.98% is achieved. Our results indicate that the laser fabrication of hybrid SERS sensor offers an efficient and cost-effective approach for ultratrace molecular detection.

Functionalization of freeform curved surfaces by shaped femtosecond laser pulses in the propagation axis

With ultrashort pulse durations and ultrahigh peak intensities, ultrafast lasers can create different types of micro/nano-structures to functionalize the processed surface with new properties. However, the applications of this method on freeform surfaces are still limited by the short length of a laser focusing spot and complex control of the 3D moving trajectory in the fabrication process. In this paper, we overcome this problem by shaping the on-axis intensity along the propagation axis using the spatial light modulator. By designing the phase mask, we increased the length of the stable-intensity zone (intensity fluctuation < 10%) by more than 3 times compared to that of an unshaped Bessel beam. The energy deposition was also optimized to be less than 2% fluctuation based on simulations. Using this method, we fabricated micro/nano structures on 3D surfaces at different fluences and demonstrated various properties including colorization, anti-reflection, and hydrophobicity in large height range. We demonstrated the applications of the proposed method in creating hydrophobicity on complex freeform syringe tip surfaces. This improved the minimum manipulatable volume of a liquid droplet to 2 times smaller compared with untreated syringe, thus greatly extending its performance for micro-droplet manipulation. This method offers an alternative approach for reliable and affordable freeform curved-surface processing.

Functional Superhydrophobic Surfaces with Spatially Programmable Adhesion

A superhydrophobic surface that has controllable adhesion and is characterized by the lotus and petal effects is a powerful tool for the manipulation of liquid droplets. Such a surface has considerable potential in many domains, such as biomedicine, enhanced Raman scattering, and smart surfaces. There have been many attempts to fabricate superhydrophobic films; however, most of the fabricated films had uniform adhesion over their area. A patterned superhydrophobic surface with spatially controllable adhesion allows for increased functions in the context of droplet manipulation. In this study, we proposed a method based on liquid-crystal/polymer phase separation and local photopolymerization to realize a superhydrophobic surface with spatially varying adhesion. Materials and topographic structures were analyzed to understand their adhesion mechanisms. Two patterned surfaces with varying adhesion were fabricated from a superhydrophobic material to function as droplet guides and droplet collectors. Due to their easy fabrication and high functionality, superhydrophobic surfaces have high potential for being used in the fabrication of smart liquid-droplet-controlling surfaces for practical applications.

Partial Leidenfrost Evaporation-Assisted Ultrasensitive Surface-Enhanced Raman Spectroscopy in a Janus Water Droplet on Hierarchical Plasmonic Micro-/Nanostructures

The conventional methods of creating superhydrophobic surface-enhanced Raman spectroscopy (SERS) devices are by conformally coating a nanolayer of hydrophobic materials on micro-/nanostructured plasmonic substrates. However, the hydrophobic coating may partially block hot spots and therefore compromise Raman signals of analytes. In this paper, we report a partial Leidenfrost evaporation-assisted approach for ultrasensitive SERS detection of low-concentration analytes in water droplets on hierarchical plasmonic micro-/nanostructures, which are fabricated by integrating nanolaminated metal nanoantennas on carbon nanotube (CNT)-decorated Si micropillar arrays. In comparison with natural evaporation, partial Leidenfrost-assisted evaporation on the hierarchical surfaces can provide a levitating force to maintain the water-based analyte droplet in the Cassie-Wenzel hybrid state, i.e., a Janus droplet. By overcoming the diffusion limit in SERS measurements, the continuous shrinking circumferential rim of the droplet, which is in the Cassie state, toward the pinned central region of the droplet, which is in the Wenzel state, results in a fast concentration of dilute analyte molecules on a significantly reduced footprint within several minutes. Here, we demonstrate that a partial Leidenfrost droplet on the hierarchical plasmonic surfaces can reduce the final deposition footprint of analytes by 3-4 orders of magnitude and enable SERS detection of nanomolar analytes (10^-9 M) in an aqueous solution. In particular, this type of hierarchical plasmonic surface has densely packed plasmonic hot spots with SERS enhancement factors (EFs) exceeding 10⁷. Partial Leidenfrost evaporation-assisted SERS sensing on hierarchical plasmonic micro-/nanostructures provides a fast and ultrasensitive biochemical detection strategy without the need for additional surface modifications and chemical treatments.

Circular ripple patterns on silicon induced by bubble-diffracted femtosecond laser pulses in liquid

We report on a new technique of silicon surface nanostructuring in liquid with a pair of Gaussian-shaped femtosecond laser pulses. The bubble, generated in liquid near the molten silicon surface by the first pulse, serves as a dynamic microscale obstacle for spatial modulation of the intensity profile of the second pulse following at a certain delay via scattering processes. As a result, the circular ripple patterns with anomalously high surface-relief modulation, undersurface annular nanocavities, and interfacial smoothness are produced at the surface. The possibility of the control over the specific pattern through the laser intensity variation is shown.

Rapidly fabricating a large area nanotip microstructure for high-sensitivity SERS applications

Here, we propose a novel nanotip microstructure which can be easily fabricated through a simply Reactive Ion Etching (RIE) process combined with anodic aluminum oxide (AAO) membranes. When combined with Ag coating and annealing on the surface of micro-sized nanotip arrays, the as-formed Ag-nanoparticles (Ag-NPs)/Si-nanotip hybrid structure exhibited a significantly high enhancement factor and highly sensitive surface enhanced Raman scattering (SERS) for rhodamine 6G molecules. The nanotip microstructure showed a sharp curvature with an apex diameter which significantly affected the SERS results. The Ag-NPs/Si-nanotip hybrid structure verified a very prominent "hot spot" effect that exists around the nanotip structures, which contributed mainly to an enhanced SERS signal with an enhancement factor (EF) of 1.6 × 10⁶. Moreover, the Ag-NPs/Si-nanotip hybrid structure demonstrated superior sensitivity, with obvious featured Raman peaks even when the concentration was as low as 10^-10 M. Our work demonstrated a feasible way to prepare a novel nanotip microstructure with a highly localized surface plasmon resonance response which could be feasibly applied for highly sensitive and reproducible SERS applications.

Hybrid superhydrophilic-superhydrophobic micro/nanostructures fabricated by femtosecond laser-induced forward transfer for sub-femtomolar Raman detection

Raman spectroscopy plays a crucial role in biochemical analysis. Recently, superhydrophobic surface-enhanced Raman scattering (SERS) substrates have enhanced detection limits by concentrating target molecules into small areas. However, due to the wet transition phenomenon, further reduction of the droplet contact area is prevented, and the detection limit is restricted. This paper proposes a simple method involving femtosecond laser-induced forward transfer for preparing a hybrid superhydrophilic-superhydrophobic SERS (HS-SERS) substrate by introducing a superhydrophilic pattern to promote the target molecules to concentrate on it for ultratrace detection. Furthermore, the HS-SERS substrate is heated to promote a smaller concentrated area. The water vapor film formed by the contact of the solution with the substrate overcomes droplet collapse, and the target molecules are completely concentrated into the superhydrophilic region without loss during evaporation. Finally, the concentrated region is successfully reduced, and the detection limit is enhanced. The HS-SERS substrate achieved a final contact area of 0.013 mm², a 12.1-fold decrease from the unheated case. The reduction of the contact area led to a detection limit concentration as low as 10^-16 M for a Rhodamine 6G solution. In addition, the HS-SERS substrate accurately controlled the size of the concentrated areas through the superhydrophilic pattern, which can be attributed to the favorable repeatability of the droplet concentration results. In addition, the preparation method is flexible and has the potential for fluid mixing, fluid transport, and biochemical sensors, etc.

SERS based detection of multiple analytes from dye/explosive mixtures using picosecond laser fabricated gold nanoparticles and nanostructures

Surface enhanced Raman spectroscopy (SERS) is a cutting edge analytical tool for trace analyte detection due to its highly sensitive, non-destructive and fingerprinting capability. Herein, we report the detection of multiple analytes from various mixtures using gold nanoparticles (NPs) and nanostructures (NSs) as SERS platforms. NPs and NSs were achieved through the simple approach of laser ablation in liquids (LAL) and their morphological studies were conducted with a UV-Visible absorption spectrometer, a high resolution transmission electron microscope (HRTEM) and a field emission scanning electron microscope (FESEM). The fabricated NPs/NSs allowed the sensitive and selective detection of different mixed compounds containing (i) rhodamine 6G (Rh6G) and methylene blue (MB), (ii) crystal violet (CV) and malachite green (MG), (iii) picric acid (explosive) and MB (dye), (iv) picric acid and 3-nitro-1,2,4- triazol-5-one (explosive, NTO) and (v) picric acid and 2,4-dinitrotoluene (explosive, DNT) using a portable Raman spectrometer. Thus, the obtained results demonstrate the capability of fabricated SERS substrates in identifying explosives and dyes from various mixtures. This could pave a new way for simultaneous detection of multiple analytes in real field applications.

Femtosecond Laser-Induced Underwater Superoleophobic Surfaces with Reversible pH-Responsive Wettability

Wettability-switchable surfaces have become a research hotspot because they can exhibit different superwetting states. In this paper, the copper surfaces with pH-responsive underwater-oil wettability were prepared by femtosecond laser treatment and subsequent chemical modification. The resultant surfaces showed underwater superoleophobicity in alkaline solutions but quasi-superoleophilicity in acidic solutions. The contact angles of an underwater-oil droplet on the resultant surfaces could be reversibly tuned between 157° and 12° by changing the pH of aqueous solutions. Such switchable wettability is ascribed to the modification of the alkyl and carboxylic acids groups on the laser-structured surfaces. The as-prepared surfaces have both oil-resistance and oil-collection abilities by selectively showing underwater superoleophobicity and superoleophilicity. The smart surfaces with pH-responsive oil wettability will have important applications in controlling the oil behavior in water.

Optical Field Enhancement in Au Nanoparticle-Decorated Nanorod Arrays Prepared by Femtosecond Laser and Their Tunable Surface-Enhanced Raman Scattering Applications

Various Au nanostructures have been demonstrated to have an enhanced local electric field around them because of surface plasmons. Herein, we propose a novel method for fabricating Au nanoparticle-decorated nanorod (NPDN) arrays through femtosecond laser irradiation combined with Au coating and annealing. The nanorod cavities strongly confined light and produced an enhanced optical field in response to Au nanoparticles (NPs) introduction. The nanogap and diameter of the fabricated Au NPs significantly affected the surface-enhanced Raman scattering (SERS) performance and could be simultaneously tuned with thickness-controllable Au films and substrate morphologies. The resulting Au NPDN substrate was observed to have efficient "hot spots" for tunable SERS applications. We experimentally determined that the enhancement factor of the Au NPDN substrate reached up to 8.3 × 10⁷ at optimal parameters. Moreover, the Au NPDN substrate showed superior chemical stability, with the greatest intensity deviation of 3.2% on exposure to air for 2 months. This work provides a promising method to fabricate tunable plasmonic surfaces for highly sensitive, reproducible, and chemically stable SERS applications.

Hierarchical Ag nanostructures on Sn-doped indium oxide nano-branches: super-hydrophobic surface for surface-enhanced Raman scattering

Herein, we fabricated a super-hydrophobic SERS substrate using Sn-doped indium oxide (Indium-tin-oxide: ITO) nano-branches as a template. ITO nano-branches with tens of nanometer diameter are first fabricated through the vapor–liquid–solid (VLS) growth to provide roughness of the substrate. 10 nm thickness of Ag thin film was deposited and then treated with the post-annealing process to create numerous air-pockets in the Ag film, forming a hierarchical Ag nanostructures. The resulting substrate obtained Cassie's wetting property with a water contact angle of 151°. Compared to the normal hydrophobic Ag nanoparticle substrate, increase of about 4.25-fold higher SERS signal was obtained for 7 μL of rhodamine 6G aqueous solutions.

Herein, we fabricated a super-hydrophobic SERS substrate using Sn-doped indium oxide (Indium-tin-oxide: ITO) nano-branches as a template.

Tunable coffee-ring effect on a superhydrophobic surface

A tunable coffee-ring effect (CRE) that enables the patterned deposition of nanoparticles (NPs) is obtained on a designed superhydrophilic and superhydrophobic composite surface of a titanium substrate. Low-adhesion superhydrophobic surfaces with picosecond laser-induced periodic surface structure and micro-nano hierarchical structure are investigated. The NPs are not only deposited in a small area of 0.045  mm², which is 265.56 times smaller than that of the original hydrophilic surface, but also in various patterns such as triangular, rectangular, and ecliptical besides the traditional circular shape. This controllable morphology of the CRE indicates a maneuvering capability of NPs in their common preservation form of suspension turbid liquid, even when the solution concentration reaches 1 mg/mL, which is promising for NP-printed circuit boards and site-specific delivery drugs.

A dual-functional surface with hierarchical micro/nanostructure arrays for self-cleaning and antireflection

A dual-functional surface with patterned hierarchical micro/nanostructure arrays has been fabricated by femtosecond laser and thermal oxidation.