Facile in-situ fabrication of target-enhanced reusable 3D hydrogel surface-enhanced Raman spectroscopy. SERS) platform in microfluidic system. In this study, we integrated SERS and microfluidic device by in-situ growth of silver nanoparticles in hydrogel micro-posts array via automated digital micromirror device (DMD)-based maskless flow lithography technique. The hydrogel-based SERS system in microfluidic channel allowed target molecules amplification effect and real-time detection.
In addition, the 3D porous hydrogel network not only enabled compact density of Ag NPs within the overall hydrogel structure, but also easily removed the target molecules after Raman measurement, providing high sensitivity and reusability. In addition, the SERS substrate was proven to be highly reproducible by measuring Raman signal from randomly selected spots and the Raman mapping images of a single hydrogel micropost. Bright field images of (A) PEGDA, (B) PEGDA/AA, and (C) PEGDA/AA/Ag hydrogel micropost array immersed in 1 M R6G. D) UV-Vis absorption spectra and (E) Raman shift of 1 M R6G solution and hydrogel microposts of PEGDA, PEGDA/AA and PEGDA/AA/Ag post immersed in 1 M R6G.
For ten different spots within a single micropost immersed in the 10 M R6G aqueous solution, (A) the full Raman spectra and (B) the standard deviations of the Raman intensities measured at each peak position are shown. For nine different spots in each of the microarrays from three individual microfluidic devices, i.e. a series of in-plane mapping of the Raman intensity measured at 1508 cm-1 along the thickness direction.
The reusability, continuous multi-target sensing capability, and a demonstration of the practical application of the PEGDA/AA/Ag micro-posts.
List of Table
Nomenclature
Silver nitrate Ag Silver
- INTRODUCTION
- Surface Enhanced Raman Spectroscopy (SERS)
- Surface Enhanced Raman spectroscopy background research
- GHB detection system background research
- Digital micromirror device (DMD) lithography
- Research Objective
- EXPERIMENTAL
- Materials
- Microfluidic device preparation
- Fabrication process of hydrogel micro-post array
- In-situ photo reduction of Ag NPs in hydrogel micro-post array in PDMS channel
- Characterization
- RESULTS & DISCUSSION
- Fabrication of Ag nanostructured hydrogel SERS system in microfluidic system
- Characteristics of in-situ developed Ag nanoparticles in hydrogel structure
- Analyte amplification effect of hydrogel structure
- Sensitivity test of Raman substrate
- Reproducibility and reliability of Raman substrate
- Reusability test of Raman substrate
- Continuous detection of multiple targets
- Practical usage by detecting date rape drug
- CONCLUSION
- REFERENCE
The physiochemically isotropic porous 3D hydrogel micropost arrays are constructed in the microfluidic channel using polyethylene glycol diacrylate (PEGDA) and acrylic acid (AA) precursor monomer solution by quantitatively controlling DMD pattern 365 nm UV light. PEGDA/AA/Ag microposts were fabricated using a custom DMD-based maskless photolithography system. Next, the entire hydrogel micropost array was irradiated with DMD pattern 365 nm UV light (4.635 mW/cm2) for 1 ~ 5 min. DI water was flowed into the PDMS channel and the PEGDA/AA/Ag hydrogel microposts were rinsed for 10 min.
Finally, DI water was flowed into the PDMS channel and the hydrogel-based SERS micropost array was rinsed with DI water for 10 min. The in situ developed Ag nanoparticles in hydrogel micropiles showed different colors depending on the UV exposure time of 365 nm (Figure 9). The hydrogel micropost array showed blue color when exposed to 365 nm UV light for 1 min, and the yellow color was further developed after 2 min exposure to 365 nm UV light.
After exposure to 365 nm UV light for 5 min, these yellowish hydrogel micro-pillars further changed to a yellowish-brown color with two characteristic absorption bands at 430 and 622 nm associated with localized surface plasmon resonance (LSPR) of Ag nanostructures. To wash out the smaller Ag NPs, we used a wash solution - an aqueous solution containing 30 vol.% polyethylene glycol (Mw = 200 g/mol, PEG200), which is known to stabilize the dispersion of Ag10 NPs and thus facilitate the release of smaller Ag NPs sizes from micro pillars without forming agglomerates. In addition, UV-Vis absorption spectra showed the complete removal of 622 nm Ag NPs with the disappearance of the LSPR band (Figure 11B). A) Bright-field images and (B) UV-Vis absorption spectra of the Ag NPs matrix of hydrogel microcubes after washing with 30 vol. % solution of PEG 200 in DI water.
First, we flowed 1 µM R6G solution into AA-free hydrogel microposts array (PEGDA-post), AA-branched hydrogel microposts (PEGDA/AA-post), and Ag nanostructured PEGDA/AA-post. CSERS is the concentration of R6G on the Ag/AA/PEGDA hydrogel micropost array (CSERS = 100 nM) and CBulk is the concentration of R6G on the PEGDA hydrogel micropost array (CBulk = 1.0 M). For ten different spots within a single micropost immersed in the 10 µM R6G aqueous solution, (A) the full Raman spectra and (B) the standard deviations of Raman intensities measured at each peak position are shown. C) The full Raman spectra and (D) the standard deviations of Raman intensities calculated at three peak positions for each of the three microfluidic devices were shown.
To evaluate the reusability, we tested the process of continuously immersing and rinsing the Ag nanostructured SERS hydrogel micro-post array with R6G and methanol (MeOH) solution in the PDMS channel, respectively. As in Figure 20A, there was almost no color change of the Ag NPs hydrogel micropost array in each rinsing cycle indicating that the structure of this system is still maintained after 20 cycles in total. For the complete adsorption of R6G molecules on the Ag NPs hydrogel micro-post array, the R6G solution was injected for a relatively short time of 10 min at a flow rate of 1.5 e-8 m3/s, and the Raman intensity increased significantly immediately noticeable.
As a result, despite being used multiple times, the fast and efficient rinsing process enables high signal reproducibility without reducing the sensitivity of the Ag NPs hydrogel microposts. PEGDA/AA/Ag hydrogel SERS micro-posts array successfully detected GHB up to 0.08 M in the presence of 40% EtOH. In the case of a PEGDA/AA hydrogel micropost array without Ag nanostructured interior, only the Raman signals of EtOH were observed and any characteristic Raman signal of GHB was not observed.
The scalable measurement was available by fabricating physicochemically isotropic hydrogel micro-post arrays in a microfluidic device through a precisely controlled automatic DMD-based maskless flow microlithography technique.
