pH‑Responsive Ag@Polyacryloyl Hydrazide Nanoparticles: A Smart, Ultra‑Sensitive SERS Substrate for Trace Analysis

Background

Surface‑enhanced Raman scattering (SERS) provides a vibrational fingerprint that allows the identification of molecules with exceptional sensitivity, making it an attractive tool for detecting DNA, RNA, and cancer cells [1,2]. The enhancement originates mainly from the localized surface plasmon resonance (LSPR) of metal nanoparticles, which amplifies the electromagnetic field in the nanogaps known as “hot spots” [3,4]. This amplification can elevate Raman signals of adsorbed molecules to detectable levels even at sub‑nanomolar concentrations [5–10].

While extensive research has improved SERS sensitivity through engineered nanoparticle shapes and sizes [11], controllable tuning of the SERS response remains underexplored [12–15]. Polyacryloyl hydrazide (PAH) is a pH‑responsive polymer rich in hydrazide groups that can act simultaneously as a reducing and capping agent for silver ion reduction [16,17]. The reversible swelling–shrinking of PAH under pH stimuli offers a straightforward route to modulate the distance between silver cores and analyte molecules, thereby controlling LSPR and the resulting SERS intensity.

In this study, we combine PAH and silver nanoparticles to produce Ag@PAH nanostructures without any external reagents. Using Rhodamine 6G (R6G) as a model analyte, we demonstrate that the SERS response of Ag@PAH can be finely tuned by adjusting pH, while maintaining a detection limit of 10⁻¹² M, positioning this platform as a promising smart sensor for trace detection of biological hazards and chemical reagents.

Methods

The fabrication procedure of Ag@PAH nanoparticles is illustrated in Fig. 1. In brief, 250 µL of 0.2 M AgNO₃ was added to 25 mL of 2 % (w/v) PAH aqueous solution. The mixture was stirred gently at 30 °C for 30 min, yielding a reddish‑brown solution that was purified by 24‑hour dialysis against deionized water and then concentrated by centrifugation. The resulting colloids were dispersed in deionized water and their pH was tuned to desired values using 0.1 M HCl or 0.1 M NaOH.

Schematic illustration of the preparation of Ag@PAH nanoparticles

Results and Discussion

High‑resolution TEM reveals that the silver cores are fully encapsulated by the PAH shell, forming a complete core–shell structure. The average core diameter is ~90 nm (Fig. 2a). Dynamic light scattering shows the hydrodynamic diameter varies from 192.6 nm at pH 9 to 103.3 nm at pH 4, indicating a shell thickness of 102.6 nm (pH 9) shrinking to 13.3 nm (pH 4). This behavior stems from protonation–deprotonation, charge repulsion, and hydrogen‑bonding dynamics within the PAH polymer. UV‑vis spectra display a consistent absorption peak near 423 nm, with intensity decreasing as pH rises, reflecting the influence of shell thickness on LSPR without altering the core optical signature (Fig. 2c).

a HRTEM images and size distribution of Ag@PAH nanoparticles. b pH dependence of hydrodynamic diameter. c pH dependence of UV‑vis absorption spectra.

The SERS performance was evaluated using R6G. Compared with pure R6G and PAH solutions, the addition of Ag or Ag@PAH nanoparticles produced pronounced Raman peaks at 1311, 1363, 1509, and 1651 cm⁻¹, confirming that the silver core is responsible for signal amplification (Fig. 3a). The PAH shell itself does not contribute to SERS, but its thickness critically modulates the electromagnetic field at the particle surface.

a Schematic of R6G adsorption on Ag@PAH substrates. b Mechanism of tunable SERS via PAH shell thickness at different pH.

By systematically varying pH, we observed that the SERS intensity of the 1509 cm⁻¹ peak decreases with increasing pH, correlating with the swelling of the PAH shell and the consequent reduction in electromagnetic enhancement. Enhancement factors (EFs) were calculated for the 1509 cm⁻¹ peak and ranged from 0.8 × 10⁶ at pH 4 to 4.3 × 10⁶ at pH 9, demonstrating that the substrate maintains a high EF across the entire pH window (Fig. 4b).

a SERS spectra of R6G at varying pH. b EF of R6G on Ag@PAH as a function of pH (1509 cm⁻¹). c SERS spectra of R6G at concentrations from 10⁻⁷ to 10⁻¹² M at pH 4. d Log‑linear relationship between peak intensity (1509 cm⁻¹) and R6G concentration (inset shows linear fit).

Testing a wide concentration range (10⁻⁷–10⁻¹² M) at pH 4, the substrate resolves the characteristic R6G bands down to 10⁻¹² M. A linear regression of the logarithm of concentration versus peak intensity yields y = 5.9838 + 0.3228 log (x) with R² = 0.9971 (n = 6), confirming quantitative behavior in the low‑concentration regime.

Conclusions

We have introduced a pH‑responsive Ag@PAH nanoparticle platform that offers tunable, ultra‑sensitive SERS performance without the need for external reducing or capping agents. The swelling–shrinking of the PAH shell controls LSPR and consequently the SERS signal, enabling adjustable detection thresholds. With a detection limit of 10⁻¹² M for Rhodamine 6G, this smart substrate is well suited for trace analysis of biological and chemical species in complex matrices.

Associated Content

Supporting information. Materials, instrumentation, preparation of PAH, and EF calculation method. Figure S1: ¹H NMR spectrum of PMA in CDCl₃ and PAH in D₂O (Additional file 1).

Abbreviations

EFs

Enhancement factors

EM

Enhanced electromagnetic

LSPR

Localized surface plasmon resonance

NPs

Nanoparticles

PAH

Polyacryloyl hydrazide

SERS

Surface‑enhanced Raman scattering