Background

Fingermarks are a feature unique to each individual. Due to this uniqueness and the permanent nature, they can be employed to determine the identity of person and, in case of a crime, provide vital clues about the identity of a suspect. Fingermarks are one of the universally accepted, oldest, and reliable evidence found on various kinds of items recovered from the scenes of crime. Individualization from fingermarks is based on the relative arrangement of Galton details, presents on fingertips, and considered as confirmatory in nature beyond any doubt (Champod et al. 2016; Ramotowski 2012). Fingermarks are the result of the perspiration residues released from the pores on the friction ridge skin of the digits. These prints are referred to as latent fingermarks, as they are generally invisible to the naked eye (Knowles 1978; Ramotowski 2012; Thomas 1978). Apocrine, eccrine, and sebaceous glands are the primary source of natural secretions from fingers. Numerous eccrine glands are present on the palms of hands which produce colorless sweat. It contains approximately 0.5% organic, 0.5% inorganic, and 99% water contents. Eccrine sweat consists of amino acids, proteins, sugars, choline, lactic acid, creatinine, urea, and uric acid, whereas sebaceous sweat consists of fatty acids, glycerides, sterol esters, squalene, and wax esters (Bumbrah 2017; Bumbrah et al. 2016; Kuno 1934; Scruton et al. 1975).

Various kinds of optical, physical, and chemical methods are commonly used alone or in a defined sequence to visualize latent fingermarks or to enhance the contrast and quality of developed fingermarks on diverse kinds of surfaces. Optical methods are nondestructive and employ electromagnetic radiations of the appropriate frequencies to reveal latent fingermarks. Physical methods involve the physical interaction of reagent or powder with residues of latent fingermarks. Chemical methods involve the transformation of a specific component of perspiration into a colored byproduct through a series of the chemical reaction. Selection of the method depends on condition (wet or dry), color (light, dark, multicolored), texture (smooth and rough), and nature (porous, semi-porous, nonporous) of surface on which the latent fingermark is deposited (Bumbrah 2016; Bumbrah 2017; Bumbrah and Rawat 2019; Bumbrah et al. 2016; Bumbrah et al. 2019; Bumbrah et al. 2022; Levin-Elad et al. 2017; Palak and Bumbrah 2019; Ramotowski 2012).

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Powder dusting, ninhydrin (NIN), 1,8-diazafluoren-9-one (DFO), 1,2-indanedione (IND), cyanoacrylate fuming, iodine fuming, oil red O (ORO), physical developer (PD), and small particle reagent (SPR) are frequently used conventional methods to develop latent fingermarks on various types of surfaces of forensic importance. These methods usually involve the interaction of fingerprint composition with amino acids, fatty acids, lipids, and/or proteins present in residues of latent fingermarks (Champod et al. 2016; Ramotowski 2012). Powder dusting is the oldest physical method to detect fresh latent fingermarks on dry, smooth, nonporous surfaces. This method is incapable of develo** aged and wet latent fingermarks and requires experienced hands and use of toxic powder formulations (Sodhi and Kaur 2001). NIN, DFO, and IND are chemical methods of develo** fresh and aged latent fingermarks on dry, porous surfaces. However, these methods are not suitable for develo** wet latent fingermarks, require posttreatment steps to enhance the contrast of developed prints, and prone to destroy the writings present on the surface of paper substrates due to the presence of toxic solvents in their working formulations (Bumbrah and Rawat 2019; Jasuja and Singh 2009; Ramotowski 2012; Sodhi and Kaur 2001). Cyanoacrylate fuming is the most suitable chemical method to develop latent fingermarks on dry and wet nonporous surfaces. But this method suffers from some practical limitations including poor contrast, use of toxic cyanoacrylate, and pre- and posttreatment procedures (Bumbrah 2017). Iodine fuming is the simplest and rapid method to develop latent fingermarks on porous surfaces. However, this method is not suitable for develo** latent fingermarks on wet surfaces. The chief limitation of this method is that developed fingermarks are not permanent in nature and fade with passage of time or on exposure to heat (Jasuja and Singh 2009). ORO and PD methods are frequently used to develop latent fingermarks on wet and porous surfaces. Extensive use of toxic solvents, laborious multistep development process, and limited contrast are some of disadvantages associated with these methods (Bumbrah et al. 2019; Chung et al. 2019; Sodhi and Kaur 2016). SPR method is used to develop latent fingermarks on wet, nonporous surfaces. However, lack of fluorescence, poor contrast, and inability to develop identifiable fingermarks on multicolored surface are major limitations of conventional SPR formulation (Bumbrah 2016).

Most of the limitations of these conventional fingermark development techniques can be resolved by using quantum dots (QDs). QDs are ultrasmall size (usually 1–10 nm) semiconducting nanomaterials that are quantum confined. QDs are characterized as zero-dimension species. QDs exhibit size unique optical properties due to tunable band gap and quantum confinement effects. Depending on their size and composition, QDs give strong fluorescence under UV, VIS, or NIR irradiation. Size-tunable QDs significantly improve the resolution of technique than conventional methods used to develop latent fingermarks (Chung et al. 2019; Ramotowski 2012). It is observed that only three kinds of QDs (CdTe, CdS, and CdSe) constitute 80% of the publications on their application in latent fingermark development (Kanodarwala et al. 2019). Menzel et al. (2000) reported the first use of QDs in develo** latent fingermarks. Photoluminescent CdS/dendrimer nanocomposites solution was used to enhance the contrast of cyanoacrylate fumed fingermarks on aluminum foil and polyethylene. In a similar study, ** et al. (2008) used photoluminescent CdS/PAMAM quantum dots (CD-24), in solution form, to improve the quality of cyanoacrylate fumed fingermarks on tin foil. Dilag et al. (2009) used chitosan encapsulated photoluminescent CdS quantum dots (CD-19) suspension for the detection of latent fingermarks on aluminum foil. Liu et al. (2010) used water-soluble multicolored fluorescent CdTe quantum dots to develop fingermarks on sticky side of adhesive tapes. Gao et al. (2011) used fluorescent positively charged CdTe QDs (CD-16) for develo** fingermarks on aluminum foil, marble, glass, polymer, rubber, and transparent polypropylene. It was observed that positively charged CdTe QDs shows superior detection capability than negatively charged CdTe QDs.

Carbon dots (CD’s), also called as carbogenic nanoparticles, are carbon-rich nanomaterials having a size of less than 10 nm. Due to its small size, they are quantum confined which makes them luminescent when irradiated with UV light. The color and intensity of luminescence depend on the diameter, shape, and surface state of CD’s (Tuerhong et al. 2017).

In recent years, the utility of CD’s, as a fingermark reagent, in develo** latent fingermarks has increased due to their high aqueous solubility, color tuneability, ease of functionalization, and low toxicity. Rich sources of carbon such as amino acids, citric acid, graphite, mushrooms, starch, or peels of pomelo and orange are frequently used as precursors in the synthesis of CD’s due to their easily availability and cost-effectiveness (Li et al. 2016; Lu et al. 2012; Prasannan and Imae 2013). Various methods such as electrochemical, microwave, pyrolysis, and solvothermal are routinely used to prepare CD’s (** et al. 2008; Liu et al. 2019a, b; Ren et al. 2018; Wang et al. 2014; Wang et al. 2018a, b,

Applications of carbon dots in develo** latent fingermarks

Li et al. (2016) used starch-based fluorescent CD’s (CD-1) to develop fresh and aged (1 year old) fingermarks on aluminum foil, coin, copper foil, zinc sheet, iron sheet, and stainless ruler. The developed fingermarks produce high contrast, color tunability, and blue fluorescence when irradiated with 365-nm UV light. The use of starch-based fluorescent CD’s over cyanoacrylate fuming, iodine fuming, and titanium dioxide powder was suggested due to high sensitivity, better contrast, less background interference, and intense fluorescence produce by these CD’s. CD-1 are useful for develo** fingermarks on dark and multicolored surfaces. Fernandes et al. (2015) used fluorescent silica-based CD’s (CD-2) to develop fresh latent fingermarks on metal, glass, and soft drink bottle foil. These CD’s possess color tuneability and are non-toxic. The developed fingermarks appear blue, green, or red when irradiated with violet, blue, and green light and f background surfaces as shown in Fig. 1 (a-e). Automated fingerprint identification system (AFIS) was used to evaluate the quality of developed fingermarks in a quantitative manner, and it was observed that 71 minutiae were detected and matched in fingermarks developed with CD-2. In contrast, only 65 minutiae were detected and matched in fingermarks developed with conventional white powder as shown. Therefore, the use of silica-based CD’s over conventional white fingermark powder was recommended due to enhanced image quality and optimal contrast on radically different backgrounds.

Fig. 1
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Fluorescence microscopy images of fingermarks developed with hybrid nanopowder (0.7 wt% C-dot—silica) on a glass slide under (a) violet, (b) blue, and (c) green excitation wavelength. A number of fluorescence images (captured at 100 magnification) have been merged via the Photoshop software to create the larger images displayed. Photoluminescence spectra (under different excitation wavelengths) of aqueous dispersions containing (d) 13 μg/ml CD’s and (e) 13 μg/ml CD’s in the presence of (150 times higher concentration) silica nanoparticles (Fernandes et al. 2015) (Reproduced from Chem. Commun., 2015, 51, 4902 with permission from the Royal Society of Chemistry)

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Prabakaran and Pillay (2020) used cost-effective, non-toxic, fluorescent nitrogen functionalized CD’s coated zinc oxide nanoparticles (N-CD’s/ZnO NPs) (CD-3) to develop fresh and aged (4 weeks old) latent fingermarks on aluminum-based surfaces such as rod, foil, and sheet and compact disc, black mat, magazine paper, iron disc, and white marble. The developed fingermarks show high contrast and blue fluorescence when irradiated with 365-nm UV light. It is strongly recommended the use of nitrogen functionalized CD’s coated zinc oxide nanoparticles to develop the latent fingermark on these surfaces over commercially available titanium dioxide and zinc sulfate powders. Zhao et al. (2019) synthesized and used multicolored fluorescent nitrogen-doped cationic CD’s (CD-17) to develop fingermarks on aluminum foil, glass, paper, optical mouse, metallic alloy, and porcelain. The synthesized cationic CD’s can be used to perform quantitative and qualitative analysis of fingermarks using multicolor fluorescence emission of cationic CD’s. These CD’s show excitation-dependent multi-fluorescence. The developed fingermarks show third-level ridge details. Due to their additional superior solid-state optical properties, improved contrast, and biocompatibility, use of cationic CD’s over traditional methods of latent fingermarks development was suggested. These cationic CD-17 are used as fluorescence-based imaging probes and therefore do not react or contaminate the evidence at crime scene. Wang et al. (2018a, b, 1). This may be attributed to the simple and fast application procedure in powder form, and unlike the solution form, there is no need of costly and toxic solvents. This makes powder dusting a convenient, cost-effective, and simple method to apply CD’s to latent fingermarks bearing surfaces as shown in Fig. 7.

Table 1 Details of various reported carbon dots (CD) used to develop latent fingerprints
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Fig. 7
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Various surfaces in reported work

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Although it is observed that some studies try to follow the standardization protocol for latent fingermark reagent, but not even a single study compares the results of CD’s processing with benchmark techniques. In addition to this, mechanism of interaction between CD’s and residues of latent fingermarks should be explored to improve the selectivity and quality of developed fingermarks. The use of CD’s for detecting latent fingermarks has not been fully explored to date, and more studies need to be performed in order to address these issues.

In that scenario, the forensic examiners can be released from the need to carry and use a number of different powders to accommodate a variety of backgrounds at the crime scene as shown in Table 2. CD’s can be readily used in the crime scene, following standard forensic procedures.

Table 2 Type of surfaces on which carbon dots (CD) were used to develop latent fingerprints
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Conclusions

In recent years, nanoparticles gained a lot of interest for the development of latent fingermarks. This is because they show lot of favorable characteristics such as small size, high contrast, facile and low-cost synthesis, and color tuneability that helps to overcome the limitations of the conventional reagents, namely low sensitivity, low selectivity, high background interference, poor contrast, and low resolution. In this present study, we review the use of CD’s in latent fingermark analysis. CD’s represent a rapidly growing class of nano-emitters that may help the creation of new and effective fingermark development method for law enforcement and criminal justice. The advantage of using these CD’s primarily lies in their relative non-toxicity as compared to inorganic quantum dots and cheaper production costs. CD’s were used as reagents for the fingermark development on several surfaces such as printing paper, aluminum foil, copper, glass, rubber, leather, tin foil, plastic, weighing paper, currency coins, reagent bottle cap, and furniture. On review of the reports, it is observed that CD’s have a huge potential to be used to develop latent fingermarks worldwide by the crime scene investigators, fingermark experts, and police personnels as an affordable reagent with a quick response time on a variety of surfaces.