Fabrication of Tattoo Paper-Based SERS Devices and Pesticides Sensing on Fruit Surfaces

As the desire for a healthy life increases, the demand for healthy food is also increasing. Therefore, the market for eco-friendly agricultural products is growing, and a monitoring system is needed to certify eco-friendly agricultural products. In order to detect pesticides on fruits, methods such as chromatography, mass spectrometry (MS) and enzyme-linked immunosorbent assay (ELISA) are mainly used. These methods are accurate and have high sensitivity, but they require pretreatments and take a long measurement time. Thus there are many limitations to use directly in fruit farms. To solve this problem, we developed patch-type SERS substrates, attaching them to the surface of the fruit simply, and then measuring pesticides by using SERS. This method does not require cumbersome pretreatment and is suitable for use in fruit farms as it is a non-contact method.<br/>We have fabricated tattoo-type surface-enhanced Raman scattering (SERS) substrates using the inkjet printing method. Silver nano ink was applied on a commercialized tattoo paper and heated to convert silver ions to silver nanoparticles. The nanoparticles formed rough nanoclusters with a size of about 100 nanometers, which is an excellent shape to amplify the electromagnetic field for SERS. The silver nanocluster was printed as a pattern with a diameter of 2 mm, and the tattoo paper-based SERS substrate is useful to cut and attach on fruits in orchards.<br/>As with the general method of using tattoo film, water was applied to the surface of the fruit, a tattoo-type SERS substrate was attached to the surface, and dried for about several seconds. After that, the carrier film was detached, leaving the SERS layer and transparent tattoo layer. The pesticide molecules on the fruit are dissolved in the water used in the attachment process and permeate between the silver nanoparticles, allowing the SERS substrate to operate effectively. In order to detect pesticides in actual fruit farms, the SERS substrate must be robustly attached to the fruit surface. Considering the external force that can be applied in fruit farms, 25 kPa pressure with 10 mm/s speed was applied on the SERS substrate on the fruit surface. No damage or deformation to the SERS particles was found. When the tattoo-type SERS substrate is attached to the surface of the fruit, the SERS material layer is attached to the surface of the fruit, and the tattoo layer is on top of it. The tattoo layer is transparent so that the laser for measurement can pass through it, and it also protects the SERS layer.<br/>The silver ink for the SERS layer consists of a mixture of ethylene glycol and silver. Because of that, large SERS peaks were observed at 1345 cm^-1 and 1500-1600 cm^-1 and they correspond to amorphous carbon sp² D-band and G-band. These background signals should be removed because they would reduce the sensitivity by acting as an interfering signal when measuring pesticides. It is usually removed by heating at high temperatures, but the tattoo layer is made of polymer and not suitable to be heated. Thus we removed the ethylene glycol by applying a laser. As the laser exposure time was increased, the carbon SERS peak from ethylene glycol decreased.<br/>Thiram is one of the most widely used pesticides in fruit farms. We dropped 1 mM thiram solution to the surface of apples and mandarins. The SERS substrate was attached to these fruits and the SERS spectra were measured. In both apples and mandarins, large peaks were observed at 1381 cm^-1 which corresponds to CN stretching mode and symmetric CH₃ deformation mode of thiram.<br/>Since the tattoo-type SERS substrate can be easily attached to any place, not just fruits, it is expected to be widely used not only for measuring pesticides but also for sensing other bio or chemical molecules by attaching the human or animal body.