SERS-active NB inks supplemented with 4,4’-bipyridine (4bpy) and 4,4’-vinylene bipyridine (BPE) were synthesized. Here, the NB inks are consisted of gold NP (AuNP)-embedded 4bpy or BPE. Aqueous suspensions of the AuNPs were synthesized by the standard citrate reduction protocol. Accordingly, we mixed the 4bpy or BPE reporter molecules with the NBs. The 4bpy NBs, BPE NBs, 4bpy NB ink, and BPE NB ink are shown in Fig. 2(a)(i)-(iv), respectively. The SERS spectra of the NB inks are shown in Fig. 2(b). The Raman vibration mode of 4bpy is classified into two modes: an in-plane vibration mode that vibrates only in the plane of the pyridine ring and an out-of-plane vibration mode that has a vibration component perpendicular to the pyridine-ring surface. The peak at 1020 cm− 1 is the ring expansion and contraction vibration mode υring, and the peaks at approximately 1080, 1230, and 1300 cm− 1 correspond to υring + γCH, γCH, and γCC + γCH, respectively. The peak at approximately 1580 cm− 1 corresponds to the molecular expansion and contraction vibration mode of 4bpy.36,37 Because BPE has a longer carbon chain structure between the pyridine rings than 4bpy, the γCH and γCC + γCH modes are not as close as those of 4bpy, and they are observed at relatively distant positions. From this comparison of the Raman spectra, 4bpy and BPE can be distinguished and act as a reporter molecule. We also confirmed that both NB inks maintained the stability of the SERS activity at room temperature for more than 4 months.
Paper coated with 4bpy NB ink is shown in Fig. 3(a). With the naked eye, it cannot be determined that the paper has some type of coating, as shown in Fig. 3(a). It is only possible to recognize that the ink is applied by touching in locations where the ink is and is not applied. Similar results were obtained with the BPE ink. An image of the square frame in Fig. 3(a) magnified with an optical microscope at the site where the NB ink is applied is shown in the insert of Fig. 3(a). Even with an optical microscope, it cannot be determined where the NBs are located. As shown in the insert of the optical micrograph of the paper surface appeared to be almost uniform. These results are consistent with the similar results of our previous study.30
To further investigate the characteristics of the coated ink, we performed microscopic Raman spectroscopy to evaluate the distribution of the NBs on the paper. A Raman mapping image of a 1000 µm \(\times\) 1000 µm region at a Raman shift of 1600 cm− 1 in 10 µm steps was performed. There was a spatial distribution in the Raman signal. It is considered that this is because the focus was slightly different in a wide observation area owing to the slight bending of the paper. Therefore, it is considered that the bright spots, derived from the strong SERS signals are almost uniformly spatially distributed. To carefully understand the local SERS spot distribution, a magnified SERS mapping was performed. The magnified Raman mapping image (a 100 µm × 100 µm region measured at a Raman shift of 1600 cm− 1 in 1 µm steps) is shown in Fig. 3(b). The Raman spectra at positions A and B of Fig. 3(b) are shown in Fig. 3(c), respectively. From the Raman mapping image, the region of high SERS intensity at 1600 cm− 1 was randomly distributed ad lumps of ~ 8–15 µm in diameter. The relatively well-spread SERS intensities are generated by the following three contributions: (1) relatively large agglomerates consisting of several NBs were mixed in the ink and non-uniform spatial distribution of the agglomerates occurred, (2) the ink soaked into the porous paper and diffused to form lumps, and (3) the laser spot of the Raman microscope that we used for this measurement was several micrometers in diameter. It was found that the ink could be very evenly applied and the NBs were uniformly and randomly distributed in space. This is very important because it means that reverse engineering in not possible. Furthermore, because printing is possible, it is expected that this method can be used in various applications as described below.
Next, different NB inks (4bpy and BPE NB inks) were stamped on paper, as shown in Fig. 4 (a). However, whether the paper was stamped or not cannot be determined by the naked eye, as shown in Fig. 4(a). When microscopic Raman mapping was performed at 1618 cm− 1, a “うし” pattern appeared, as shown in Fig. 4(b), where, “うし” are Japanese kana characters and denote a cow. Next, when the Raman shift was set to 1201 and 1298 cm− 1, it was found that “う” and “し” separately appeared, as shown in Fig. 4(c) and 4(d), respectively. The Raman spectra of the “う” and “し” inks are shown in Fig. 4(e). Recalling the Raman spectra in Fig. 2(b), the inks for the “う” and “し” stamps are BPE and 4bpy NB inks, respectively. Therefore, from the imprint “うし”, imprint, only the BPE print pattern can be observed when read at 1201 cm− 1 (Fig. 4 (c)). Conversely, when it is observed at 1298 cm− 1, only the print pattern of only 4bpy can be observed (Fig. 4 (d)). Furthermore, from the result in Fig. 4 (b), the overwritten patterns of both inks can be observed when read at 1618 cm− 1. From the above results, the characteristics of the plasmonic seal and stamp were shown.
As described the above, chemical PUF stamps can provide reliable authenticity tracers as plasmonic stamps. Finally, to demonstrate the capability of the chemical PUFs for practical use, we created an authenticity system that digitizes from the Raman signals read by a mobile Raman spectrometer and smartphone. This system will be a gatekeeper that connects cyberspace and physical space to perform authenticity judgment, as schematically shown in Fig. 5. During commodity circulation, the chemical PUFs can be labeled, read, digitized, and authenticated at each stage, including by the manufacturer, distributor, retailer, and the end user in the supply chain. If a digital label does not match any data in the blockchain, it will be considered a fake. To demonstrate the authenticity judgment based on the digital chemical PUFs, we developed and integrated the software into a smartphone with an android-based operating systems (OS). The results of reading some chemical PUFs using a portable Raman spectroscope, digitizing them, and then communicating with the authenticity judgment system of the smartphone are shown in Fig. 5. If the result is true, “True” is shown on the display of the smartphone. Conversely, if it is false, “False” is displayed on the smartphone.
To construct a more flexible and dynamic supply chain, including data distribution and use, the real space where humans live is the mechanism equivalent of the blockchain mechanism realized by peer-to-peer communication in cyberspace, as schematically illustrated in Fig. 6, which needs to be implemented in the real world (physical space). In addition, a system that authenticates by linking information in cyberspace and physical space will be indispensable. That is, to ensure the reliability of the function of accurately “transcription” between physical space and cyberspace, or, in other words, to convert sensors and electronic signals that convert reality to data to physical motion. Nanotags are chemical PUFs, and they are expected to be used as high- security tags that represent numerical information. The Raman spectrum, which is a chemical fingerprint, becomes physical and numerical data as the Raman shift. In other words, the Raman spectrum is converted from real space to cyberspace by quantifying chemical PUFs. The reverse is also true. Overall, our system based on the combining with chemical PUFs, and cloud authentication can provide an effective way to achieve Society 5.0. 38