| Fingerprint is the papillary pattern on the fingertip.Since the 19th century,fingerprint has been regarded as one of the most effective evidence for personal identification in the forensic investigation due to its universality,unique,immutable,and retaining traceability of touching objects.In many common cases,latent fingerprints(LFPs)are invisible to the naked eye and require some specific treatments to visualize them.Although numerous methods have been exploited to reveal LFPs,it remains challenging to acquire enough minutiae or display the level 3 features of fingerprints on difficult surfaces(such as coins,dark or fluorescent material objects,skin,etc.)and problematic fingerprints(such as mutilated,blurred,overlapping,deformed fingerprints),which cannot fully meet the needs of practical applications yet,and affect the subsequent fingerprint identification and recognition in a certain degree.Moreover,investigators often find fingerprints left behind at crime scenes.It is significant for criminal investigation and analysis to assess the fingermark aging process according to the relationship between physical ridge features and chemical composition of fingerprint residues over time,which contributes to searching and selecting relevant evidence,and screening out irrelevant information.Nevertheless,the composition of fingerprint is liable to factors including donor features,deposition conditions,substrate properties,and environment conditions.All the influences mainly contribute to unreliable outcomes of age estimation,which is a long-standing unsolved problem in forensic science.In this thesis,we devote to exploring the high-resolution development method of fingerprint multi-level features and reliable methods of fingerprint aging time based on scanning electrochemical microscopy and flexible material,the main research contents are as follows:(1)Wet nitrocellulose(NC)membrane for the multi-level feature visualization of various LFPs and gender determination.A clear fingerprint pattern could be presented on the membrane after a fingertip touch owing to the difference in light transmittance between the ridge residues and the wet-NC-membrane background.In contrast with the furrow area(flexible membrane substrate)with good light transmission,a clear fingerprint pattern could be presented on the membrane.Compared with traditional methods,this method could provide higher resolution fingerprint images to accurately extract level 3 features.The wet NC membrane method could also be used to extract and preferentially develop fingerprint samples,and has good compatibility with other methods as well.Moreover,the sensitivity and contrast of fingerprint visualization could be further improved by modifying the NC membrane with chitosan and ink,which could be used to transfer and develop LFPs from various substrate surfaces with high resolution.In addition,due to the excellent feasibility and reproducibility of level 3 features extracted by the wet-NC-membrane,frequency distribution of the distance between adjacent sweat pores(FDDasp)could be used to effectively distinguish the fragmentary fingerprints,which provided valuable additional information for the fragmentary LFP identification.Finally,144 fingerprints were extracted from 30 donors of different gender,and the number of sweat pores was statistically analyzed.The statistical results indicated that females had a higher average sweat pore density(115/9 mm2)than males(84/9 mm2)which was expected to be a useful parameter for gender identification.(2)A label-free strategy for imaging blood fingerprints(BFPs)has been developed by scanning electrochemical microscopy(SECM).The principle is that hexaammineruthenium chloride([Ru(NH3)6]Cl3)was chosen as a redox mediator to react with the electroactive hemichrome(HC)present in the BFP ridge deposits.This provides a sharp contrast of the current response at the tip of the electrode between ridges and furrows and results in an electrochemical image of the BFP with the high resolution.In addition,this approach has high sensitivity,with the ability to electrochemically image latent BFPs diluted 1000 times.Moreover,for the first time a commercial polyvinylidene fluoride(PVDF)membrane was used instead of adhesive forensic tape to lift the BFPs from common glass and banknote surfaces with complex patterns,and the lifted BFPs could also be efficiently imaged by this electrochemical method,which expands the range of applicable objects.(3)The electrochemical behavior of hemoglobin was detected by SECM to estimate the time since deposition(TSD)of BFP.In this approach,the estimation of TSD is based on the changes of tip current depending on the reaction between the tip-generated reductant[Ru(NH3)6]2+ and hemichrome(HC)formed during bloodstain aging,whose intensity depends on the concentration of HC.Lateral scans were performed over 12 bloodstains with different TSD in two set of timelines(4.5-61 h and 2-144 h),and the results showed that the current increased with TSD.The current data of these bloodstains were fitted by a logarithmic function to establish a current-TSD aging fitting curve,providing a quantitative basis for TSD estimation.Finally,the method effectively identified fresh(TSD:5 h)and aged BFPs(TSD:77 h)deposited on the PVDF membrane that were difficult to be distinguished by optical methods,while the TSD of BFP was evaluated based on the current-TSD aging fitting curve which showed the errors of the evaluated results was 9.1%and 2.4%,respectively.(4)Alkaline phosphatase(ALP)activity was measured by SECM to evaluate the TSD of BFP.P-Aminophenyl(PAPP)added to the solution was hydrolyzed by ALP to generate the electroactive mediator p-Aminophenol(PAP),which was oxidized to p-quinoneimine(PQI)by a probe at a bias potential of+0.35 V(vs.Ag/AgCl)to produce a current response that can be used to evaluate the activity of ALP.The relationship between ALP concentration and the current was indirectly obtained by detecting samples of different concentrations of ALP deposited on PVDF membranes to provide a quantitative basis for estimating the TSD of BFP.By analyzing the results of lateral scanning currents of different TSD bloodstains on glass and PVDF membrane surfaces,we found that the current signal decreased with TSD,indicating that the ALP activity decreased with the TSD.Furthermore,the reaction kinetic constant kf of ALP in bloodstains with different TSDs was extracted to establish the quantitative relationship between ALP activity and TSD.Similarly,the TSDs of fresh and aged BFPs with no significant visual difference were evaluated by SECM.The results showed that the TSDs of fresh BFP and aged BFP deposited on PVDF membrane were 56.4 h and 116.9 h with an error of 5.0%and 2.4%,respectively.The TSDs of fresh BFPs and aged BFPs deposited on glass were 49.6 h and 144.1 h with an error of 6.5%and 2.4%.The accuracy of the evaluation results of BFPs deposited on PVDF membrane was slightly better than those of hemoglobin-based method. |
PDF Full Text Request