Preparation Of Solid Carbon Quantum Dots And Application Of Latent Fingerprint Development

Carbon quantum dot（CD）is a new member of the photoactive carbon family that has emerged in recent years.Due to its excellent optical properties,easy surface modification and low cost of raw materials,CD has attracted great attention.In criminal investigation,Latent fingerprint（LFP）is the most common physical evidence,and powder development is the most popular method for LFP development.Portable CD powder shows great potential in LFP development,but there are still some problems to be solved.For example,CD powder inevitably has the risk of blowing dust,which poses a threat to the health of users.The effect of aggregation caused quenching（ACQ）often leads to luminescence self-quenching of solid-state CD.The long-wavelength region that is more suitable for fluorescent labels lacks fluorescence emission.The Photoluminescence quantum yield（PLQY）is low.Firstly,in view of the dust blowing problem of the most popular powder method for detecting LFP,a user-friendly and safe preparation process of magnetic and fluorescent CD powder was proposed.In this paper,a layer-by-layer assembly route of magnetic fluorescent nanocomposites is presented.The solid-state fluorescence quantum yield of the final product is45.04%,the particle size is 120 nm,and the saturation magnetization was 6.4 emu/g.Under the irradiation of 370 nm,the maximum emission peak of the Fe₃O₄@Si O₂-CD_（n）powder is located at 434 nm,and with the increase of the number of assembled layers,the fluorescence quenching effect is positively correlated with distance.FTIR,XPS and contact angle measurements further confirmed its abundant carboxyl,amide and hydrophilicity.More importantly,the novel composite powder has been successfully applied as an ideal fluorescent probe for LFP development with high-quality images in Level 2 and Level 3,indicating suitable physical adsorption and enhanced chemical bonding between print residues and developing powders.The AFIS also proves the superiority of the Fe₃O₄@Si O₂-CD_（n）powder over some traditional powders.It is worth noting that the surface of the prepared bifunctional nano-powder has abundant hydrophilic groups,which can better adsorb moisture in the air and avoid the fingerprint powder from diffusing into the user’s respiratory tract.Secondly,in view of the shortage of long-wavelength emitting solid CD,a simple and convenient preparation of multicolor CD solutions was proposed using neutral red as the carbon and nitrogen sources by solvent hot method,layering and extraction procedure.After polyvinyl pyrrolidone（PVP）as the cross-linking agent is doped into as-prepared solutions,red-,orange-and yellow-emitting CD powders are finally fabricated.The results show that the absolute fluorescence quantum yield（QY）of solid carbon dots is 6.33%,16.5%and 43.6%,respectively.Through surface functional group and photoluminescence（PL）property studies compared with solution and powder state of CD,the Cross-linking Effect Emission（CEE）brought by PVP is confirmed in charge of the high solid state QY of CD powders.In addition,the conjugate structure of neutral red and increasing hydrophobicity are believed to be responsible for the long wavelength emission of CD powders.Notably,multicolor imaging of latent fingerprints was successfully achieved on complex background surfaces under 365 nm UV illumination.Finally,this paper obtained a user-friendly and safe preparation process of magnetic and fluorescent composite CD powder,and a simple synthesis process of multicolor long wavelength luminous CD powder.The interaction between magnetic carbon dot powder and fingerprint residue and the dominant effect of CEE on CD solid QY is higher than liquid QY.These carbon dot powders are successfully used for LFP imaging based on different objects.The results of this paper provide guidance for latent fingerprint development.