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Discussion The results showed a surprising mi2 of DNA in stains even at extreme environmental conditions (35–65 °C, 100% relative humidity), provided the stains had been dried prior to incubation. We had expected a rapid decay of DNA at high relative humidity but observed, on the contrary, that even long DNA fragments were amplifiable after month of incubation. The appearance of fungal/microbial growth on some of the stains at 100% humidity may explain the decrease in amplifiable DNA observed for these stains. However, the average environmental humidity is well below 100% even during wet seasons, e.g. the average value is 90% for the winter month in Denmark  and it does not exceed 82% at any time of the year in tropical Darwin, Australia . Therefore, even in humid climates simple air-drying seems to be adequate for the preservation for month of DNA in stains and reference samples to be analysed for STRs and SNPs. This may be especially attractive to developing countries that want to implement modern DNA-profiling techniques but find the purchasing of expensive sampling kits prohibitive.
Introduction Water-soluble fluorescent metal nanomaterials (NMs), such as nanoparticles (NPs) or nanoclusters (NCs), with small size about several nanometers containing few atoms have attracted much attention due to their unique size-dependent properties such as fluorescence and catalysis in the past years. Among them, noble metal NCs with sizes less than 2 nm, approaching the Fermi wavelength of the electron, have been widely utilized as emerging fluorophores on account of their lower toxicity and better photostability compared to traditional organic dyes and quantum dots (Berti and Burley, 2008, Guo et al., 2010, Gwinn et al., 2008, Houlton et al., 2009, Kennedy et al., 2012, Li et al., 2011). Compared with noble metals such as gold and silver, fluorescent copper NMs (CuNMs), including copper NPs (CuNPs) and copper NCs (CuNCs), show more extensive prospects in science and industry due to the lower cost and richer earth stock of metallic Cu, attracting lots of scientists to explore their synthesis and application (Anzlovar et al., 2007, Cho et al., 2001, Chrimes et al., 2013, de Oliveira et al., 2007; Du et al., 2005; Godovski, 1995; Hall et al., 2000; Helmy et al., 2013; Kang et al., 2016; Kharenko et al., 2005). To achieve the practical use of fluorescent CuNMs (Brouwer, 2011, Cao et al., 2014, Chen et al., 2014), various template ligands usually containing sulfur or nitrogen functional groups that can interact with copper ions (Chrimes et al., 2013, Cummings et al., 1980, Eaton, 1988, Han et al., 2017, Helmy et al., 2013, Kang et al., 2016, Liu et al., 2013a, Qing et al., 2013a, Rotaru et al., 2010, Vilar-Vidal et al., 2010, Wang et al., 2017a) have been explored to improve the stability of fluorescent CuNMs. Among all the capping ligands, DNA due to its diversity and ability to recognize molecules has been widely utilized as the efficient templates for the formation of CuNMs. The first report on the selective formation of fluorescent CuNPs (Rotaru et al., 2010) was based on the double-stranded DNA (dsDNA) and serials of biosensing platform including metal ions, small molecules, proteins, DNA and RNA. Subsequently, Liu et al. and Qing et al. found that single-stranded DNA (ssDNA) could also be used to synthesize CuNPs (Liu et al., 2013a, Qing et al., 2013a), which further extended application of DNA templated CuNMs (DNA-CuNMs). From then on, more and more DNA templates with different structures were well-designed and investigated for the probes for the sensing (Peng et al., 2018, Qing et al., 2017, Wang et al., 2017a, Wang et al., 2016c). For example, grafting the T-loop to the ds-DNA forming hairpin structure or utilizing crowded effect to improve the fluorescence of DNA-CuNMs were demonstrated for high-performance sensing (Qing et al., 2017, Wang et al., 2017a, Wang et al., 2016c). In the initial stage, the optical feature of DNA-CuNMs was often investigated as the single readout and lots of enzyme reaction and DNA based hybridization chain reaction have been introduced to broaden the application. Recently, to achieve the accurate results, coupling different metal NCs and other DNA fluorescent switcher (Chen et al., 2017a, Chen et al., 2016, Wu et al., 2016) with DNA-CuNMs provided a new strategy for sensing. Meanwhile, with the development of in situ eletrochemical reduction for the formation of DNA-CuNMs, the electrochemical-related features of DNA-CuNM have been excavated including the larger electrical resistance, the electrochemical stripping signal, the electrochemiluminescence (ECL) response of DNA-CuNMs and the Cu(II) based catalyzed oxidation of eletrochemical probes (Hu et al., 2017, Liao et al., 2018, Wang et al., 2015e, Zhou et al., 2018). Moreover, the chemical calalytical properties of the DNA-CuNMs, such as the enzyme-mimic properties of Cu(II) generated by acid treatment of DNA-CuNMs were also exploited (Borghei et al., 2018, Mao et al., 2016). And the applications have been extended from sensing to the logic gate construction, and staining.