Surface enhanced Raman scattering (SERS) is a multifunctional detection technology widely used in chemical and biological molecules. It has the advantages of high detection sensitivity, no sample treatment, nondestructive testing, etc. SERS technology can realize ultra-low concentration molecular detection or even single molecule detection. As a flexible substrate material, paper is different from the traditional rigid substrate. The rigid substrate is fragile, which greatly limits the application of plasma nanostructures. The flexible substrate can be easily cut into different shapes and sizes to meet the needs of non-planar, flexible, and other applications. At the same time, as paper has the characteristics of easy access and low cost, there are many explorations in the preparation of paper-based SERS substrates, and there are various preparation methods for paper-based SERS substrates, such as pen on paper, spray preparation, and other preparation methods. However, the preparation process of these paper-based SERS substrates is relatively cumbersome, and it is difficult to meet the different pattern design requirements under different environmental conditions. In this paper, the Ag-paper-based SERS substrate is prepared by inkjet printing, and the high-performance paper-based SERS substrate is prepared by selecting the optimal parameters.

In this study, an Ag-paper-based SERS substrate is prepared by inkjet printing on hydrophobic A4 paper. Firstly, a brown-green silver sol solution with an average particle size of 58.6 nm is prepared by the Lee preparation method, and the parameters of silver nitrate, sodium citrate, and heating time are set. Silver ink is prepared as follows. Silver sol is centrifuged and ultrasonically operated, the supernatant is removed, and the silver sol washing process is repeated twice. According to the silver sol concentration multiples of 10, 20, 33, and 50, supernatants of 50, 53, 54, and 55 mL on the surface are removed, respectively. The absolute ethanol and glycerin are added according to the configuration scheme of silver ink, and the silver ink is filtered with a polytetrafluoroethylene (PTFE) membrane filter (with a pore size of 0.2 mm), so as to ensure the working fluency of the silver ink. After that, the paper receives hydrophobic treatment with a 20% mixed hexanol solution of dodecene succinic anhydride (DDSA). Next, the silver ink is put into the ink cartridge for a few minutes and printed on the hydrophobic A4 paper. Finally, the prepared Ag-paper-based SERS substrate is used for Raman detection of probe molecules.

The prepared Ag-paper-based SERS substrate can be mass-produced rapidly. At the same time, it is cheap, and the preparation scheme is simple. The operator can quickly start to prepare and apply and thus realize multi-molecule detection. Through the optimal selection of silver ink multiples and the number of printing layers (Fig. 2), when the silver ink concentration is 50 times, and the number of printing layers is 7, the prepared Ag-paper-based SERS substrate has high Raman enhancement performance, with the maximum enhancement factor of about 1.92×10⁹ and the relative standard deviation (RSD) of 14.3% (Fig. 3). After hydrophobic treatment of A4 paper, Raman detection sensitivity is greatly improved (Fig. 4). This is because after hydrophobic treatment, the contact angle of liquid droplets on the paper surface has been greatly improved, which leads to more silver nanoparticles per unit surface area. Therefore, Raman detection intensity has been enhanced. Paper is a flexible material. The flexible characteristic experiment (Fig. 5) shows that the prepared Ag-paper-based SERS substrate also shows excellent uniformity after bending, which proves that the Ag-paper-based SERS substrate has strong stability.

In this paper, the silver ink is printed on the hydrophobic A4 paper surface by an inkjet printing method, so as to serve as a flexible SERS substrate. By studying the influence of different silver ink multiples and printing layers on Raman detection sensitivity, it can be found that when the silver ink concentration is 50 times, and the number of printing layers is 7, the detection concentration of R6G molecules on SERS substrate is as low as 10^-10 mol/L, and the maximum enhancement factor is about 1.92×10⁹. At the same time, the SERS substrate has excellent uniformity, and the optimal RSD calculation result is 14.3%. The flexibility of Ag-paper-based SERS substrate is studied on the apple surface, and the detection of multi-molecules is realized. The method has the advantages of low price and mass production. Finally, according to the scanning electron microscope (SEM) results of the SERS substrate, the electromagnetic field enhancement characteristics of the SERS substrate are calculated by using finite-difference time-domain (FDTD) software. According to the numerical simulation and experimental results, the optimal number of printing layers is 7.