Microfluidic synthesis of high-valence programmable atom-like nanoparticles for reliable sensing

Abstract

Synthesis of programmable atom-like nanoparticles (PANs) with high valences and high yields remains a grand challenge. Here, a novel synthetic strategy of microfluidic galvanic displacement (μ-GD) coupled with microfluidic DNA nanoassembly is advanced for synthesis of single-stranded DNA encoder (SSE)-encoded PANs for reliable surface-enhanced Raman scattering (SERS) sensing. Notably, PANs with high valences (e.g., n-valence, n = 12) are synthesized with high yields (e.g., >80%) owing to the effective control of interfacial reactions sequentially occurring in the microfluidic system. On the basis of this, we present the first demonstration of a PAN-based automatic analytical platform, in which sensor construction, sample loading and on-line monitoring are carried out in the microfluidic system, thus guaranteeing reliable quantitative measurement. In the proof-of-concept demonstration, accurate determination of tetracycline (TET) in serum and milk samples with a high recovery close to 100% and a low relative standard deviation (RSD) less than 5.0% is achieved by using this integrated analytical platform.

Fig. 1. Illustration of the stepwise fabrication of SSE-encoded PANs in the microfluidic system. The detailed models illustrating the microfluidic GD (μ-GD) reactions between Ag⁺ and the silicon surface (1^st reaction), affinity reactions between SSEs and nanoparticles (2^nd reaction) and DNA hybridization reactions between SSEs and their complementary sequences (3^rd reaction) are shown in dotted frames.

Fig. 2. Characterization of cAgNPs. (a) Schematic diagram of the microfluidic device. (b) Photo of the microfluidic device. Schematic illustrations of silver ion (Ag⁺) diffusion and the formation of core AgNPs (cAgNPs) at the silicon surface in the (c) microfluidic or (d) bulk GD system. Simulations of silver ion diffusion in the microchamber with (e) or without microfluidic flow (f). (g) Top-view and (h) side-view SEM images of cAgNPs prepared in μ-GD systems and corresponding (i) histograms of size distribution. (j) Top-view and (k) side-view SEM images of cAgNPs prepared in bulk-GD systems and corresponding (l) histograms of size distribution.

Fig. 3. Characterization of SSE-encoded PANs. (a–c) SEM images of SSE-encoded PANs prepared in the microfluidic system at different magnifications. Scale bars are 400 nm, 200 nm and 100 nm. For clarity, the AuNPs in (b) and (c) are colored using Adobe Photoshop. (d) Statistical analysis of n-valence PANs (n < 7, n = 7–9, n = 10–12 and n > 12) corresponding to cAgNPs with different diameters. The total number of cAgNPs is 300. Simulations of SSE (polyA–P1) diffusion in the microchamber with (e) or without (f) microfluidic flow using COMSOL. (g) Raman spectra collected from the as-prepared PANs in the microfluidic system and corresponding SERS mapping results of the 730 cm⁻¹ SERS peak (h). (i) Raman spectra collected from the as-prepared PANs in the bulk system and corresponding SERS mapping results of the 730 cm⁻¹ SERS peak (j).

Fig. 4. FDTD simulation of the normalized EM-field intensity distribution (IEI²/IE₀I²) for PAN, AgNP and Au–AgNP: (a) XZ and (b) XY plane view of a single PAN on the silicon substrate. (c) XZ and (d) XY plane view of a single AgNP on the silicon substrate. (e) XZ and (f) XY plane view of a single Au–AgNP (without DNA as a linker) on the silicon substrate.

Fig. 5. Sample loading via microfluidics. (a) Schematic illustration of sample loading via a pipette gun. (b) Photo image (I) of the detection zone and the corresponding SERS mapping (II) of the Raman peak at 1364 cm⁻¹ in the same location by the conventional sample loading method. (c) Corresponding Raman spectra of 10⁻⁴ M R6G directly dropped onto the prepared PANs. (d) Schematic illustration of sample loading via microfluidics. (e) Photo image (I) of the detection zone and corresponding SERS mapping (II) of the Raman peak at 1364 cm⁻¹ in the same location by the microfluidic method. (f) Corresponding Raman spectra of 10⁻⁴ M R6G pumped onto the prepared PANs by microfluidics.

Fig. 6. PAN-based automatic analytical platform for TET detection. (a) Schematic illustration of the PAN-based automatic analytical platform for TET detection. (b) Raman spectra of functionalized PANs, pure Raman dye of Cy3 and polyA30. (c) Raman spectra of functionalized PANs with different TET concentrations. (d) The linear fittings of the ratiometric signal (I₁₅₈₆/I_BG, I₁₅₈₆/I₇₃₀) versus the logarithmic concentration of TET. BG means background. (e) Raman spectra and (f) corresponding ratiometric signals (I₁₅₈₆/I₇₃₀) from the functionalized PANs loaded in milk and serum samples (n = 16) spiked with different concentrations of TET.