Highly Sensitive SERS Substrates from Aligned, Chemically Etched Silver Nanowire Monolayers

Background

Surface‑enhanced Raman scattering (SERS) has become a cornerstone of modern analytical chemistry, offering rapid, non‑destructive, and ultra‑sensitive detection of a wide range of molecules [1,2,3]. The technique relies on the amplification of Raman signals by several orders of magnitude, especially when analytes occupy the so‑called “hot spots” – nanoscopic regions of intense electromagnetic (EM) fields near sharp edges, roughened surfaces, or interparticle junctions.

Silver nanowires (AgNWs) are attractive SERS candidates due to their large surface area and high crystallinity [4,5]. However, conventional AgNWs generate hot spots predominantly at their termini [6], leading to an uneven distribution of SERS intensity and limiting reproducibility. When nanowires are brought into close proximity, the EM field in the interstitial gaps is predicted and experimentally observed to be markedly enhanced [7,8]. Prior work demonstrated that aligning AgNWs increases Raman intensity and that the intensity depends on the polarization of the incident laser [9]. Various assembly strategies—Langmuir–Blodgett, layer‑by‑layer, external field, liquid–liquid interface—enable dense, ordered arrays of nanowires, but the hot‑spot density remains limited if the nanowires themselves are smooth.

Surface roughening—via metal deposition, chemical etching, or decoration with nanoparticles—has proven effective at creating additional hot spots along the nanowire axis [20–22]. Chemical etching, in particular, can transform smooth nanowires into roughened structures with a dramatically higher SERS signal [23–25]. Yet most studies focus on isolated, roughened nanowires or on random assemblies; the combination of alignment and roughening has not been thoroughly explored. In this work we present an aligned monolayer of chemically etched AgNWs, fabricated by a simple three‑phase interfacial assembly technique. The resulting substrates exhibit an unprecedented detection limit of 1×10⁻¹¹ M for RB, along with excellent reproducibility (RSD ≈ 12 %).

Materials

Silver nitrate (AgNO₃, 99.8 %, Sinopharm), rhodamine B, poly(vinylpyrrolidone) (PVP, Mw ≈ 58 000), copper(II) chloride dihydrate, ethylene glycol (EG), 30 % hydrogen peroxide, 25 % ammonia solution, and Milli‑Q deionized water (resistivity > 18 MΩ·cm) were all analytical‑grade reagents used without further purification.

Methods

Synthesis of Ag Nanowires

AgNWs were synthesized via a modified polyol route. EG (100 mL) was heated to 160 °C, then 1.5 mL of 4 mM CuCl₂·2H₂O in EG was added. After 15 min, 30 mL of 0.4 M PVP in EG was injected, followed by a syringe‑pumped addition of 30 mL of 0.2 M AgNO₃ (1.5 mL min⁻¹) under magnetic stirring. After ~30 min, the solution turned opaque gray, indicating AgNW formation. The product was cooled, washed twice with acetone and water to remove excess PVP and EG, and dispersed in ethanol.

Etching of Ag Nanowires

Fresh etchant (ammonium hydroxide:30 % H₂O₂ in a 9:1 volume ratio) was prepared on ice. 200, 300, or 400 µL of this etchant was mixed with 4.5 mL of 1 mg mL⁻¹ PVP aqueous solution. 500 µL of 5 mg mL⁻¹ AgNW suspension was then added under vigorous stirring. The solution changed color instantly and evolved gas; the reaction completed within seconds and was allowed to proceed for an additional 5 min.

Fabrication of Aligned Ag Nanowire Substrates

5 mL of the as‑synthesized or etched AgNW suspension was spread on the surface of 25 mL of chloroform in a glass vessel. A 1 mL drop of acetone was added slowly, producing a mirror‑like film at the interface. The ordered film was transferred onto silicon wafers and labeled S0 (no etchant), S1 (200 µL), S2 (300 µL), and S3 (400 µL).

Characterization

Morphology was examined by SEM (JEOL JSM‑7001F) and AFM (JEOL JSM‑7600F). UV–vis spectra were recorded on a Shimadzu UV 2450. Crystallinity was assessed by XRD (X’Pert Powder) using Cu‑Kα radiation (λ = 0.15405 nm) over 30°–90° 2θ.

Raman Spectroscopy

SERS spectra were collected on a HORIBA Jobin Yvon system with a 633 nm Ar⁺ laser (1.7 mW, ~1 µm spot). Each spectrum was accumulated for 20 s. RB solutions (10⁻⁷ – 10⁻¹¹ M) were deposited (0.02 mL) onto 7 × 7 mm² substrates, forming a ~65 mm² circular spot. The laser power and acquisition time were held constant across all measurements.

Results and Discussion

Fabrication and Morphology

Figure 1 outlines the fabrication workflow: etching → interfacial alignment → transfer. As‑synthesized AgNWs are smooth with an average length of 19.5 µm and diameter of 120 nm. SEM images (Figure 2a) show a densely packed, parallel array. After etching, the surface develops pronounced waviness (Figure 2b–d), with the diameter decreasing from ~120 nm to ~80 nm as the etchant volume increases. AFM confirms a substantial increase in surface roughness and a reduction in diameter (Figure 3).

UV–vis spectra (Figure 4) reveal a shift of the transverse plasmon peak from 377 nm (as‑synthesized) to 370 nm (etched), accompanied by a broadening of the full width at half maximum, consistent with increased roughness and reduced wire diameter.

XRD patterns (Figure 5) show five prominent fcc silver peaks (111, 200, 220, 311, 222) with identical positions before and after etching, indicating that the crystalline structure remains intact.

SERS Performance

SERS spectra of 1×10⁻⁷ M RB (Figure 6a) display characteristic bands at 920, 1110, 1210, 1260, and 1330 cm⁻¹. Etched substrates exhibit markedly stronger signals, with intensities increasing with etchant volume, attributable to the creation of abundant hot spots along the wire length. Polarization studies (Figure 7) confirm that parallel‑polarized excitation enhances the signal on aligned films, and the effect is stronger for etched wires, underscoring the role of surface roughness.

Quantitative analysis (Figure 6b) demonstrates detectable Raman peaks down to 1×10⁻¹¹ M RB, establishing a detection limit of 10⁻¹¹ M. The log‑log plot (Figure 6c) shows a linear relationship (R² ≈ 0.98) for concentrations below 10⁻⁸ M, while saturation occurs at higher concentrations due to surface coverage limits.

Reproducibility was evaluated by measuring six random spots at 1×10⁻⁹ M RB (Figure 6d). The relative standard deviation of the 1647 cm⁻¹ band is only 12 %, well below the 20 % threshold commonly accepted for SERS substrates.

Conclusion

We have demonstrated that chemically etched, aligned AgNW monolayers combine high surface roughness with ordered architecture, yielding SERS substrates that deliver an ultralow detection limit (10⁻¹¹ M) and excellent reproducibility (RSD ≈ 12 %). The preserved fcc crystallinity, coupled with the generation of abundant hot spots along the wire axis, underpins the superior performance. These substrates represent a robust, scalable platform for homogeneous and ultrasensitive SERS detection in chemical and biological sensing.