Emerging Inkjet-Compatible Anti-Counterfeiting Inks Based on Microfluidic-Synthesized NIR PbS/CdS Quantum Dots

The lead sulfide (PbS) quantum dot (QD) material has excellent near-infrared (NIR) emissions. It is possible to synthesize core-shell-structured PbS/CdS QDs using the cation exchange method (Pb²⁺ → Cd²⁺), which enhances mass transfer and reaction uniformity and increases their stability. However, the large-scale synthesis of core-shell PbS/CdS QDs is needed to fulfill the industrial needs. Here, we used a microfluidic device to synthesize PbS and PbS/CdS QDs in enabling accurate control of flow rates, mixing, temperature, and residence time, which are critical for achieving uniform nucleation and controlled growth of QDs. This results in highly monodisperse nanocrystals with consistent optical properties. PbS QDs could be tuned by wavelengths from 1200 to 1600 nm and with full width at half-maximum (fwhm) varied from 213 to 272 nm. The core-shell PbS/CdS QD production was completed within 5 min, the wavelength was blue-shifted by 150 nm, and the fwhm was maintained at 223 nm with superior stability. The fabricated NIR QDs are then used to produce NIR emissive ink compatible with commercial-grade inkjet printers. An oily hydrophobic ink was produced by first dissolving the NIR QDs in an octadecene-octane mixture and then adding polyethylene to increase stability. After that, the ink was assessed by inkjet printing onto different paper substrates. Under the illumination of the blue light source, the printed ink was invisible under ambient and could be decrypted only when photographed by an NIR camera. This new NIR anti-counterfeiting identification mechanism represents a covert optical security approach, offering a novel, low-cost, and high-security solution for anti-counterfeiting applications.