Novel Methods For Visualization And Electrochemical Imaging Of Latent Fingermarks

Fingermarks have long been used as the gold standard for personal identification in forensic investigation,secret-involved system,registration and social security.Most fingermarks encountered in daily life and forensic science are called latent fingermarks,which are not readily visible to the naked eye.Generally,fingerprint techniques are divided into four categories:visualization,image capture,identification and automated system techniques.Although innumerable methods have been exploited for fingermark visualization,it still remains challenging to acquire fingermark information in a technically simple,rapid,and easy handling way.Additionally,research interest has been recently directed to detect the chemical components in fingermark residues.The additional secretions about the donor in fingermark provide molecular information in addition to the ridge pattern,such as the age,gender,medical history,and possible drug habits.The most common fingermark capture method is taken by optical devices,such as camera.But photographic equipment requires higher fingermark quality,suitable on-site light and skilled photographer.The aim of this thesis is to establish a prospective study to solve the problems above in fingermark visualization and capture,in order to provide new methods and ideas for the application of scanning electrochemical microscopy in forensic science.Firstly,a high-resolution and universal method for the rapid visualization of latent fingermarks on a broad variety of surfaces has been achieved by simply combining hydrophilic cellulose membrane with dye aqueous solution in 3-5 s.It provides observation of latent fingermarks on cellulose membrane surface with high definition of level 2 and level 3 details used for personal identification purposes.It is also highly-efficient for visualizing the latent fingermarks lifted from various common(glass,carton,ceramic cup)and problematic(banknote,human skin and leather)surfaces by using transparent adhesive tape.Moreover,this approach is easily used for aged fingermarks and natural fingermarks as well.This approach is a simple,rapid,non-destructive,safe,low-cost,universal and high resolution method compared with conventional approaches,so it demonstrates good potentiality in individual identity validation related applications.The forensic tape lift method is the most common technique used for collection of fingermark residues in various crimes.But in cases where important evidence is fragile,the viscosity of forensic tape may damage the substrate and/or LFMs.In order to solve this problem,a high-resolution and universal method for simultaneous transfer and visualization of latent fingermarks on a broad variety of surfaces for preservation has been achieved by simply combining nitrocellulose(NC)membrane or polyvinylidene fluoride membrane with methylene blue(MB)solution/silver staining solution.This approach is fast and simple that the transferred latent fingermarks can be clearly visualized within 1 min.It is highly-efficient for visualizing the latent fingermarks transferred from various common(glass,advertising paper,ceramic cup)and problematic(human skin and leather)surfaces.Moreover,the proposed method is able to simultaneously transfer and visualize fingermarks by using the gold nanoparticles functionalized with antibody and silver staining to provide additional chemical information in fingermark than just identification.This approach is safe,low-cost,universal,high-resolution and non-destructive to substrate and following analysis,so it demonstrates good potentiality in individual identity validation related applications and metabolite detection.Finally,a label-free electrochemical method based on scanning electrochemical microscopy(SECM)has been developed to chemically image latent fingermarks with high resolution.SECM imaging can avoid the interference of the background-color with high sensitivity.This new in situ electrochemical technique operates in a surface-interrogation mode for the detection of fingermark residues(such as squalene)on surface of NC membrane.A SECM tip generates reduced methyl viologen(MV2+)MV·+ radical from the reversible redox mediator MV2+ that reacts chemically with the components in fingermark residues when MV·+ diffuses to the NC membrane,which is positioned at a short distance from it.The reaction between the MV·+ radical and the components in residues generates more MV2+ to accelerate the redox reaction.Therefore,it provides positive feedback tip current until the fingermark residues are consumed completely.On the contrary,the tip current decreases monotonically with the tip approaching the blank or furrows of the substrate because MV2+ diffusion hindrance.Changes in the tip current during lateral scans used for electrochemical imaging reflect the topology of the fingermark residues on the surface.In this way,label-free sebaceous fingermarks,methylene blue-enhanced sebaceous fingermarks,natural fingermarks.blood fingermarks,and aged fingermarks can yield high-resolution images.The SECM images of the human fingermarks are of comparable or better quality than those obtained by conventional optical methods.This technique provides a brand new method for fingermark detection that enables both identification of an individual and recognition of the human secretions.