Synthesis of Palladium Nanoclusters and Application in UV Surface-Enhanced Raman Spectroscopy

Abstract

Palladium is an attractive but underdeveloped ultraviolet SERS material whose enhancement is strongly coupled to the surface chemistry. We introduce a lipid-confined, dry-state thermal reduction route that converts a drop-cast Na2PdCl4/DOPC precursor on Si into a tunable Pd nanocluster layer, in which DOPC acts as an active determinant of nanocluster formation. Transmission electron microscopy resolves discrete Pd nanoclusters of mean diameter 1.9 ± 0.5 nm in the residual lipid, and spectroscopic ellipsometry shows a near-UV plasmonic response that overlaps the 325 nm excitation line. Raman analysis of Pd−Cl vibrations identifies a narrow processing regime in which metallic Pd dominates. Under 325 nm excitation, the adenine UV-SERS response is strongest on substrates prepared at 100 °C, where enhancement factors reach 102 to 103 and the limit of detection for the diagnostic band is 29 μM; at higher preparation temperatures, the response falls as the nanoclusters coalesce and their plasmon resonance shifts away from the excitation wavelength. The substrates expose low-frequency markers of chemical bonding: a Pd−N stretch at 251 cm−1 for adenine and Pd−S markers at 342−356 cm−1 for sulfur-containing adsorbates. Density functional theory assigns the adenine spectrum to the N7H tautomer chemisorbed through its N3/N9 ring nitrogens, reproducing the 251 cm−1 Pd−N marker. A wavelength-dependent comparison indicates a substantial electromagnetic contribution, while the DFT-predicted order-of-magnitude Raman increase for the key adenine mode identifies a limited chemical enhancement.