Superhydrophobic silver film as a SERS substrate for the detection of uric acid and creatinine

: Superhydrophobic silver films were fabricated by silver-mirror reaction and surface functionalization with thiol. The thiol-functionalization significantly improved the hydrophobic property of the Ag films (AFS), and their contact angle values slightly increased with the extension of a thiol alkyl chain, reaching about 160°. The surface-enhanced Raman scattering (SERS) detection capacity of these films were investigated, and AFS-Dodec showed the best substrate for R6G molecule detection with the concentration limit of 10 − 11 M. AFS functionalized with dodecanethiol (AFS-Dodec) was applied for the SERS detection of uric acid and creatinine, it exhibited good linear dependence relationship between the Raman intensity and analyte concentration in the concentration range of 5~1000 μ M.


Introduction
Uric acid is a metabolite considered as an inert end-product of purine catabolism, its concentration in blood is determined by the balance between uric acid production and excretion. It is identified as an important biomarker in urine and serum samples for metabolism abnormally, strong correlated of renal dysfunction in rheumatoid arthritis cardiovascular [1,2] and gout disease [3]. The high concentration of uric acid (>0.4 mM) is associated with patients experiencing severe preeclampsia [4]. Likewise, creatinine in serum also reflects the extent of kidney damage, such as acute kidney injury [5]. Therefore, an effective detection method is imperative for monitoring the content and change.
Surface-enhanced Raman scattering (SERS) [6,7] is a sensitive spectroscopic technique capable of providing a characteristic molecular vibrational fingerprint, even at trace concentrations. Since the first report on SERS in 1977, many efforts have been made to achieve higher sensitivity in chemical and biological detection [8,9]. It is generally regarded that SERS enhancement stems from two mechanism [10,11]: One is chemical enhancement, which is realized by charge transfer between the analytes and the SERS substrate; the other is electromagnetic enhancement, which plays the dominant role in SERS enhancement [12,13]. It is well known that metal nanoparticles such as silver and gold can lead to strong electromagnetic fields, which is termed "hot spot" in SERS measurements [14], thus nanostructured metal material with different morphologies (nanorod, nanosphere, nanocubic, nanowire, etc) were fabricated to create more hot spots [15]. In addition to the intrinsic hotspot density, the surface state of the SERS substrates also considerably influences the SERS effect. The common SERS substrates have hydrophilic surface, which makes the analyte solution spread over the whole surface, resulting in the low detection limit even with high density-hotspots. Interestingly, the hydrophobic substrate can cause great shrinkage of the solution during the solvent evaporation process, and the analyte molecules are concentrated molecules to b The hydro chemical com prepare hydro contact angle micro/nano su cannot wet th smooth silver better water c great enhance Because s heavy metal c purpose of dodecanethiol to improve th oxidizing nan hydrophobic m of silver film obtained silve evaporated t spectroscopy

Materials
All reagents u water. Silver chloride, buta octadecanethi Crystal violet purchased fro

Fabricati
Ag film (AFS was added to to a smaller be exposed to l ophobic prope mposition of t ophilic and hyd es with 30° a urface structur he inner structu r film under th contact than th ement of Rama sulfhydryl grou can be decorat high SERS l [20]  AFS-Dodec AFS f the silver part e together, form Fig. 3(a), AFS which is much nt surface roug tive way to pre ic thiols to imp avior is obviou mage (Fig. 3(c es. the film, FTIR own in Fig. 4, A v as -CH 2 ); 1464 ymmetric and decanethiol (28 llic surface [20 expected, AFS ( Fig. 3(c)).  Fig. 6(b)) also measured t n ranging from aman peak d e 7(b) shows t cm −1 and the SERS substrat ty for the quant d the change shown in Fig  eases from 3    To investigate the practical application of this superhydrophobic silver film, AFS-Dodec was used as SERS substrate to detect uric acid and creatinine. In the case of uric acid (shown in Fig. 8(a), all samples display strong Raman peaks at 638, 811, 885, 1133, 1389 and 1608 cm −1 . The 638 cm −1 belongs to skeletal ring deformation and 1133 cm −1 is originated from C-N, more details are shown in Table 1. It is noted that SERS can equally occur for all type of molecules in a bioprobe and the appropriate select of the target Raman lines can increase signal-to-noise ratio [22][23][24], thus the peak at 1133 cm −1 was selected for further analysis of uric acid. Figure 8(b) presents the linear dependence relationship between Raman intensity of the peak at 1133 cm −1 and the concentration of uric acid, with R 2 = 0.996. Similar result was obtained for creatinine detection, shown in Fig. 9. The characteristic Raman peaks intensity increases with the increase in the concentration of creatinine, and the linear dependence relationship between Raman intensity of the peak at 681 cm −1 and the concentration of   Fig. 6(a)).
has been 6(b) and glass and t may be ted rough ins of the pectra, we e between m for the sensitive iscovered have the ed with a ree SERS d to their g and dry molecules one [25]. eriority of molecules ver-mirror film has a rough surface, and the surface becomes superhydrophobic after thiol-functionalization. By virtue of the condensation effect on the superhydrophobic surface, the silver film modified with dodecanethiol behaves as the best SERS substrate for the detection of R6G molecule with the detection limit of 10 −11 M. AFS-Dode was also applied for the detection of uric acid and creatinine, and it exhibited linear dependence relationship between Raman peak intensity and the analyte concentration range of 5~1000 μM. The study demonstrated a facile strategy to fabricate efficient SERS sensor with high detection sensitivity.

Funding
National Natural Science Foundation of China (Nos. 61575043); Natural Science Foundation of Fujian Province of China (grant no. 2016J01292); Program for New Century Excellent Talent in Fujian Province (Nos. J1-1160).