SERS detection of surface-adsorbent toxic substances of microplastics based on gold nanoparticles and surface acoustic waves

Abstract

Microplastics adsorb toxic substances and act as a transport medium. When microplastics adsorbed with toxic substances accumulate in the body, the microplastics and the adsorbed toxic substances can cause serious diseases, such as cancer. This work aimed to develop a surface-enhanced Raman spectroscopy (SERS) detection method for surface-adsorbent toxic substances by forming gold nanogaps on microplastics using surface acoustic waves (SAWs). Polystyrene microparticles (PSMPs; 1 μm) and polycyclic aromatic hydrocarbons (PAHs), including pyrene, anthracene, and fluorene, were selected as microplastics and toxic substances, respectively. Gold nanoparticles (AuNPs; 50 nm) were used as a SERS agent. The Raman characteristic peaks of the PAHs adsorbed on the surface of PSMPs were detected, and the SERS intensity and logarithm of the concentrations of pyrene, anthracene, and fluorene showed a linear relationship (R² = 0.98), and the limits of detection were 95, 168, and 195 nM, respectively. Each PAH was detected on the surface of PSMPs, which were adsorbed with toxic substances in a mixture of three PAHs, indicating that the technique can be used to elucidate mixtures of toxic substances. The proposed SERS detection method based on SAWs could sense toxic substances that were surface-adsorbed on microplastics and can be utilized to monitor or track pollutants in aquatic environments.

Fig. 1. Schematic representation of detecting polycyclic aromatic hydrocarbons (PAHs) adsorbed on polystyrene microparticles (PSMPs) using surfaced-enhanced Raman spectroscopy (SERS) based on surface acoustic waves (SAWs). (A) A 1 μL droplet consisting of 1 μm PSMPs and 50 nm gold nanoparticles (AuNPs) is loaded on the SAW chip. (B) After being aggregated and evaporated completely by the SAWs, (C) SERS signals are measured from the cluster with 785 nm laser excitation on 1 mW for 1 s.

Fig. 2. Schematically illustration of the particle aggregation on the SAW chip. (A) Interdigital transducers (IDTs) consist of 20 pairs of 10/50 nm Cr/Au electrode arrays. (B) The width of a finger and the gap between fingers is 50 μm. And the width of a pair consisting of two fingers and two gaps is 200 μm. 19.04 MHz and 19.4 V are applied to the IDTs to generate SAWs. (C) The SAWs propagate along the LiNbO₃ surface and half of the droplet should be located in the SAW propagation pathway. (D) The droplet revolves and particles in the droplet are aggregated due to internal flow induced by the SAWs.

Fig. 3. Microscopic and SEM images of aggregated 50 nm AuNPs mixed with 1 μm PSMPs adsorbed with pyrene depending on various conditions: (A) on the original LiNbO₃ surface without SAWs, (B) on the original LiNbO₃ surface with SAWs, (C) on the hydrophobic coated LiNbO₃ surface without SAWs, (D) on the hydrophobic coated LiNbO₃ surface with SAWs. Each 1 μL of the droplet was loaded on the LiNbO₃ surface.

Fig. 4. (A) Microscopic images of the aggregation and evaporation process of 1 μL droplet consisting of PSMPs and AuNPs. SEM images of (B) aggregated 1 μm PSMPs and (C) the distribution of 50 nm AuNPs on the surface of the PSMPs.

Fig. 5. (a) Raman spectrum measured from the cluster of 1 μm PSMPs without AuNPs. SERS spectra measured from the clusters of 50 nm AuNPs and 1 μm PSMPs, which adsorbed (b) no PAHs, (c) fluorene, (d) anthracene, and (e) pyrene. 10 mM PAH solution was used for the PSMPs to adsorb a PAH, respectively. Detected SERS characteristic peaks of each PAH are comparable with their own Raman characteristic peaks.

Fig. 6. (Left) SERS spectra of three PAHs adsorbed on 1 μm PSMPs at concentrations of (from a to g) 10 nM, 100 nM, 1 μM, 10 μM, 100 μM, 1 mM, and 10 mM (A) pyrene (B) anthracene and (C) fluorene. (Right) Relationship between the SERS intensity of (D) pyrene at 591 cm⁻¹, (E) anthracene at 390 cm⁻¹, (F) fluorene at 735 cm⁻¹ and the logarithm of concentrations of each PAH. Each data point represents the average value from five SERS spectra. Error bars show the sample standard deviations.

Fig. 7. SERS spectrum of a mixture of 3 PAHs (pyrene, anthracene, fluorene) with a total concentration of 10 mM adsorbed on 1 μm PSMPs. Discriminant peaks of each PAH are labeled with different symbols. Peaks with similar shifts are partially overlapped and observed as broad peaks.