# A High-Regularity Porous SERS Substrate Prepared by Two-Step Mild and Hard Anodization for Sorbic Acid Detection

## TL;DR

This paper introduces a new method to create a SERS substrate using anodization, which improves detection of sorbic acid in food products.

## Contribution

The two-step anodization method produces high-regularity AAO substrates with enhanced SERS performance for detecting sorbic acid.

## Key findings

- AAO substrates fabricated using hybrid pulse anodization show high total pore circumference and abundant hot spots for SERS.
- Sorbic acid detection achieved a limit of detection as low as 10 ppb with a calculated enhancement factor of 1.02 × 10⁵.
- Characteristic SERS peaks for sorbic acid were observed at 1640–1645 cm⁻¹.

## Abstract

Traditional colloid SERS substrates are mostly based on metal nanoparticles (MNPs), which have complex and time-consuming fabrication processes, poor structural control, and are susceptible to oxidation. As a result, solid-state SERS substrates have emerged as an effective alternative. Here, we propose using two-step mild and hard anodization to fabricate ordered anodic aluminum oxide (AAO) substrates with high total pore circumference for SERS detection. Hybrid pulse anodization (HPA) enables the fabrication of AAO at room temperature using 40 V in the first step and 40, 110, and 120 V in the second step of anodization. The different voltages applied in the second step effectively control the pore diameter, thereby achieving various nanostructures. The enhancement mechanism primarily originates from the high total pore circumference of nanostructures, which generates abundant hot spots around the pore peripherals, thereby significantly amplifying the SERS signal. Sorbic acid is a common preservative widely used in food products and employed as a test substance on high regularity AAO substrates at concentrations of 1000 ppm to 10 ppb. The resulting SERS spectra exhibited distinct characteristic peaks at 1640–1645 cm−1. The analytical enhancement factor is calculated as 1.02 × 105 at the AAO substrate prepared by 110 V with the Si substrate as the reference. By appropriately tuning the process parameters, a limit of detection (LOD) as low as 10 ppb of sorbic acid was achieved.