The use of nanomaterials has brought significant advances in various fields, including biomedical applications. However, the synthesis of these materials often involves complex and expensive processes. Metallic nanomaterials are typically synthesized by wet chemistry or photolithography. However, metallic materials with complex shapes cannot be prepared by traditional wet chemical methods [1]. Although photolithography can control the structure of the material, it requires expensive production equipment and is limited by the optical diffraction limit ^[2][3][2,3]. DNA(deoxyribonucleic acid)-guided metallization of nanomaterials has emerged as a promising technique to overcome these challenges. DNA-guided metallization of nanomaterials refers to the process of using DNA molecules as templates to guide the synthesis and assembly of metallic nanoparticles ^[4][5][4,5]. This technique offers a simple, cost-effective, and efficient way to produce a wide range of nanomaterials with tailored shapes and sizes for various applications. The use of DNA as a template for metallization provides a high level of control over the properties of the resulting nanomaterials. Notably, DNA has been introduced during the growth phase of metal nanoparticles to control and regulate the morphology of the particles [6]. In this context, thwe researchers reeview the latest developments in DNA-guided metallization of nanomaterials and their biomedical applications, as well as outline future research directions to enhance the potential of this technology.

In 1998, Braun et al. [7] proposed the use of DNA as a template to guide the growth of metallic materials. However, in the early stages of development, metallization was usually uniformly deposited along the DNA scaffold, compromising its biorecognition and addressability. To address these issues, researchers have developed several new strategies. For instance, DNA can be specifically labeled with an aldehyde group (e.g., glutaraldehyde), which acts as a local reductant on DNA and can be programmed to direct the metallization process only on selected DNA strands ^[8][9][8,9]. Moreover, the difference in affinity between DNA bases and metals can be exploited to achieve sequence-selective metallization [10]. This is because various nucleic acid bases have different affinities for metallic materials. Generally, DNA-directed metal particle synthesis involves two steps: first, metal nanoseeds bind to DNA as nucleation sites for selective metal deposition, and then metal atoms will form a continuous metal structure along the shape of DNA [4]. For gold nanomaterials, the relative adsorption affinity of DNA bases is adenine (A) > cytosine (C) ≥ guanine (G) > thymine (T) [11]. In the case of silver nanomaterials, G and C bases exhibit higher affinities for silver ions, and guanine has the highest oxidation potential among the nucleic acid bases, which leads to the specific binding of metal ions to GC-rich DNA. Upon oxidation and binding to DNA, silver atoms can gain electrons from the reduced guanine base. A series of successive cycles of oxidation and the transfer of Ag atoms from nanoparticle to DNA results in the localization of atoms at specific sites along the DNA, or the formation of a small number of atomic silver clusters near the sites of particle binding on poly(dG)-poly(dC) molecules [10].

Continuous development has shown that DNA sequence combinations can control the different morphologies of nanoparticles during their growth, and that these effects can be synergistic or competitive. The DNA used to guide the growth of nanomaterials is stable and retains its biological recognition capabilities [12]. As a result, the synthesis of nanocrystals with controlled three-dimensional structures using DNA is both feasible and appealing. The approach using DNA enables the solution-based synthesis of nanocrystals with controlled three-dimensional structures in the desired orientation, and extends the current tools available for the design and synthesis of functionally rich nanomaterials for future translational biotechnology.

The unique properties of these DNA-guided metal nanomaterials have led to their exploration in a range of biomedical applications. Metal nanoparticles have unique optical, electronic and magnetic properties [13]. DNA-directed metallization of nanomaterials can combine the unique physicochemical properties of metal nanomaterials with the biological functions of DNA for biomedical applications, especially in biodetection, biosensing, therapy or medical imaging.

2. Strategies of DNA Functionalized Nanoseeds

The formation of metal nucleation sites depends on the binding of metal ions or complexes to DNA and their subsequent reduction to form metal clusters, or on the binding of small metal particles to DNA. In the DNA-directed assembly of nanoparticles, chemical bonds are commonly used to connect DNA to nanoparticles, such as the Au-S bond commonly used in gold nanoparticles ^[14][15][16][14,15,16]. Silver nanoparticles are commonly modified with DNA using disulfide, lipoic acid, or cyclic disulfide bonds ^[17][18][19][17,18,19]. Compared to Au-S, Ag-S chemical bonding is less stable, and, hence, SH-DNA modification of silver nanoparticles is not commonly used. Chemically bonded covalent modifications possess better specificity, but they are cumbersome to pre-modify. However, when DNA is used to guide the growth of nanomaterials, it is more common to use electrostatic or ligand interactions to bind DNA to metallic nanomaterials ^{[20][21][22][23][24][25][26]}[20,21,22,23,24,25,26]. This is because the process of metallization often occurs in a solution where DNA is added to a solution containing metal ions for co-incubation [27]. The positively charged ions are attracted to the negatively charged phosphate backbone in the DNA backbone, and the DNA catalyzes their reduction, leading to the formation of metal clusters along the DNA [9]. These small clusters tend to grow into nanoparticles, and DNA strands covered with negative charges on the surface prevent aggregation. Reduction methods include chemical reduction, photochemical reduction ^[28][29][28,29], and electrochemical deposition ^[30][31][30,31]. Additionally, polymorphic DNA chains, such as polyA chains, can be rapidly adsorbed on the surface of gold nanoparticles ^[32][33][32,33], while silver can be specifically ligated to C bases (Ag-C) [34]. In other words, DNA can be modified on the surface of silver nanoseeds by intrinsic Ag-C ligands. The strong Ag-C ligand not only makes Ag-DNA couples easy to form, but also shows good stability under high ionic strength and high-temperature conditions ^[35][36][35,36].

3. DNA as Director for Nanomaterials Metallization

In this section, thwe researchers mainly focus on the use of DNA to guide the metallization of metallic nanomaterials. So far, the main discussion has been on the functionalization of nanomaterials using DNA after their synthesis, where the morphology of the nanomaterials is already determined and DNA cannot influence it. The method of seed-mediated synthesis using DNA as a director can be used to prepare nanomaterials with controlled morphology [6]. The success of this approach is mainly based on the fact that different bases of DNA have different affinities for the surface of nanomaterials, resulting in different growth rates and morphologies. For instance, taking gold nanomaterials as an example, the adsorption affinities of different bases are as follows: A > C ≥ G > T [11]. Moreover, DNA can be easily modified with various groups such as thiol groups, carboxyl groups, and amino groups, which further extends the overgrowth method to regulate the morphology of nanoparticles using nanoparticles as seeds and DNA as a guiding agent. DNA metallization can be divided into two main steps: assembling the DNA template and organizing the nanoparticles ^[37][38][37,38]. The first step involves activation, in which the negatively charged DNA binds to cations or nanoparticles through coordination or electrostatic interactions. The next step is reduction, in which the reducing agent converts the metal cations into initial clusters. The commonly used reducing agents in this process include chemical reagents (e.g., NaBH₄, ascorbic acid), UV light, and electricity ^[39][40][41][39,40,41]. Finally, in the growth step, unbound or newly introduced ions/nanoparticles are further reduced or deposited on the previous metal cluster sites [42]. The shape of the generated nanostructures is mainly influenced by two factors: the initial shape of the nano-seed and the sequence and secondary structure of the DNA. The size, orientation, and anisotropy of the crystals can be regulated by controlling nanoparticle concentration, solution ionic strength, and DNA grafting density on the particles and surface. In addition, metal nanocrystals with tunable surface plasmon resonance (SPR) can be obtained by using template molecules to guide the precise growth of metal atoms along a designed template ^{[43][44][45][46][47][48][49]}[43,44,45,46,47,48,49]. Proper metallization of DNA can improve its electrical conductivity while maintaining the geometric features of DNA nanostructures. This imparts magnetic and optical properties to DNA and expands its potential applications. DNA-guided metal nanocrystals can be precisely controlled relative to the SPR peak position, and their optical properties can be further enhanced by controlling their morphology [50]. Currently, gold is one of the most widely investigated metal nanocrystals. Different DNA templates produce nanocrystals with various morphologies and specific SPRs by regulating the growth of gold along the DNA backbone. As the number of metal branches guided by the template DNA increases, the surface of the synthesized nanocrystals becomes rougher, enhancing the plasmon resonance between the nanoparticle core and the branches, and leading to the maximum absorption redshift peak. Moreover, different sizes of metal branches due to different DNA structures also cause the maximum absorption wavelength to change ^[49][51][52][49,51,52]. Potential DNA templates have different secondary structures, such as single-stranded (ss), double-stranded (ds), hairpin (hp), and triple-junction arms (ta). Researchers have successfully synthesized several shapes of DNA-directed nanostructures, including nanoflowers, nanopolygons, nanocaps with nanobridges, and sea cucumber-like structures [53]. Some examples related to these structures are listed in Table 1.