TopUp Plasmonic Arrays for Surface‐Enhanced Raman Spectroscopy

Abstract

III in paprika powder, [ 11 ] azorubine in drinks, [ 12 ] or melamine in milk. [ 13 ] In clinical application schemes, SERS shows high potential as a point-of-care method for the online monitoring of morphine, methadone, or dopamine [ 14 ] in intravenous delivery tubing or of levofl oxacin in urine. [ 15 ] Moreover, the detection of environmentally relevant and harmful substances, i.e., arsenic (III) in lake water samples [ 16 ] or veterinary drugs such as metronidazole and ronidazole in various water sources, [ 17 ] extends the application fi eld of SERS. All of the above-mentioned application examples share the enhancement of the naturally weak Raman signal by employing nanostructured metal surfaces, so-called SERS substrates.The applied nanostructured SERS substrates are commonly generated via wet chemistry [ 18 ] and bottom-up [ 19 ] or topdown [ 20,21 ] approaches. Metal colloids can be prepared in large quantities via wet chemical reactions, e.g., the reduction of silver or gold salts to the pure metal. In general, metallic nanoparticle suspensions are characterized by easy handling and fast production in high quantities. In contrast, limitations are related to their low time stability, poor batch-to-batch reproducibility, and spectral interference originating from the reaction participants. As an alternative, laser ablation techniques are employed to fabricate metal nanoparticles without stabilization agents on their surfaces. [ 22 ] Moreover, planar substrates produced by immobilization of metallic nanoparticles based on a bottom-up technique are characterized by low homogeneity across a large area and low batch-to-batch quality.Top-down techniques can overcome some of the above-mentioned limitations by structuring thin metal layers using lithographic techniques, e.g., electron beam lithography (EBL) [ 21,23 ] or nanoimprint lithography (NIL). [ 24 ] These procedures lead to highly homogenous planar substrates with well-defi ned structures and plasmonic resonance tunabilities. However, due to the multiple processing steps required for top-down techniques, the manufacturing time for a given structure is extensive and the process is expensive. Thus, the top-down production of SERS active substrates is realized in small quantities.By an appropriate combination of bottom-up and top-down techniques, the creation of a SERS active substrate that uses the advantages and avoids the drawbacks of each applied fabrication technique becomes feasible. In principle, a sophisticated top-down approach is improved by a fast and easy bottom-up method. As an example, particle-in-cavity structures [ 25 ] were created for which the template (the cavity structure) is fabricated by nanosphere lithography (NSL) [ 25 ] following a porous By combining a bottom-up process with top-down fabricated templates, the benefi ts of both methods are joined by creating so-called TopUp plasmonic arrays for surface-enhanced Raman spectroscopy (SERS). The morphological and optical characterization of the TopUp substrates illustrates exceptional...

TopUp SERS Substrates with Integrated Internal Standard

“…This nanostructured surface was achieved by a combined electron beam lithography and lift-off process as mentioned within chapter 4 ‘Materials and Methods’. Thus, the isolated gold islands serve as seeding particles for the bottom-up synthesis of silver nanostructures [ 25 ]. The arrangement of the gold islands template structure was chosen as a closest packaging of hexagonal arranged dots in order to allow for high SERS signal intensities and homogenous distribution.…”

“…However, variations in SERS intensity due to local differences in the signal enhancement will not be accounted for. In a previous study, the potential of the TopUp SERS substrates for quantitative analysis was already introduced [ 25 ], i.e., the reliable detection of the analyte sulfamethoxazole in the nanomolar range is achieved by a homogenous signal distribution across the surface. Moreover, due to the applied measuring conditions (scanning mode), the SERS signal intensity is averaged across the surface to minimize fluctuations in the recorded SERS signal due to local changes in the plasmonic behavior.…”

“…The preparation protocol for the self-organized silver deposition on the gold template structure is described elsewhere [ 25 ]. Briefly, the gold template structure was incubated in a mixture consisting of 2 mM hydroquinone in citric buffer (pH 3.8) solution and an aqueous 2 mM silver acetate solution in the ratio 1:2.…”

“…Within this manuscript, we introduce the application of silicon as an internal standard for our recently developed TopUp substrates [ 25 ]. Here, silicon is used as supporting material for the SERS active substrate.…”

Modulating the Kinetics of Nanoparticle Adsorption for Simple and High‐Yield Fabrication of Plasmonic Heterostructures as SERS Substrates

“…Methodst of abricate such heterostructures can be broadly categorized into two groups:t op-downl ithography and bottom-up self-assembly.T he former approachc ommonlyi ncludes electronb eam lithography together with metal evaporation and lift-off/etchingt echniques. [14,17,18] Despite the highlevel accuracy and reproducibility,t he fabricated structures present polycrystallinity and surface roughness, in addition to the issues associated with the cost and throughput of the process. In ac ontrary approach, heterostructures are fabricated via abottom-up assembly of colloidal NPs in solution using different types of molecular interactions.…”