# Morphological Control: Properties and Applications of Metal Nanostructures

Authors: Supat Chupradit; M. Kavitha; Wanich Suksatan; Mohammad Javed Ansari; Zuhair I. Al Mashhadani; Mustafa M. Kadhim; Yasser Fakri Mustafa; Shafik S. Shafik; Ehsan Kianfar
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

Metal nanomaterials with special physicochemical and plasmatic properties have a wide range of applications in various fields including catalysts, plasmon devices, spectroscopy, fuel cell, and various sensors including chemical, colorimetric, and fluorescence sensors. These applications are made possible by controlling the morphology and properties of nanostructures and increasing their selectivity. Various methods have been developed for the synthesis of metal nanostructures, including the use of prefabricated patterns or hard templates such as anodic aluminum oxide and soft molds such as cetyltrimethylammonium bromide (CTAB).


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## Body

## 1. Introduction

In recent years, metal nanocrystals such as silver, gold, copper, due to their nanometer size, unique physicochemical properties, and surface plasmon behavior, have a high potential for use in various industries and technologies [1–3]. A small number of these nanocrystals have found special applications due to their special behavior in interacting with light. The study of the process of interaction between electromagnetic waves and conduction band electrons in metal nanoparticles is called plasmonic [4, 5]. Nanostructured plasmatic materials are known for their strong interaction with incoming light and free electrons. Here, metal nanostructures act as a source of conversion of light into a local electric field (electromagnetic excitation along with mass oscillations of free electrons) in metals, called localized superficial plasmonic [6, 7]. These plasmatic interactions can be controlled by changing the morphology and dimensions of metal nanostructures [8–10]. Applications related to the field of nanoplasmatic include super lenses, Raman-enhanced surface spectroscopy (SERS), molecular spectroscopy, plasmonic-enhanced fluorescence, quantum computing, energy-dependent photochemical lithography, energy-assisted plasma ionization [11–15]. Silver is more than 50 times cheaper than gold, and because of its unique physicochemical properties, it is a good choice for use in plasmatic applications [16–18]. Gold nanoparticles, depending on their size, have good surface plasmon resonance (SPR) and show SPR absorption in the visible light region. Gold nano rods, gold nano cages, and hollow nanospheres have near-infrared absorption [19–21]. Surface-enhanced Raman scattering (SERS) is more suitable for silver nanostructures with sharp corners and edges than spherical (edgeless) silver nanoparticles [22–25]. Usually, 1–10 nm metal nanoparticles have good electron and optical effects due to their free electron path [26–28]. Therefore, by controlling the structural factors, their potential applications for use in the fields of catalytic processes, electronics, photonics, sensors, medicine, etc. can be improved [29–32]. The optical properties of metal nanostructures are highly dependent on their size and shape [33, 34]. The controlled morphology of nanostructures is an important factor that has a strong impact on their unique plasmonic properties. Until now, various methods are adopted for the preparation of controlled size and shape of nanostructures [35]. In general, metal/silver salt precursor is mixed with reducing agent in the presence of stabilizing agent and control the size and shape of metal nanostructures. Past research publications show that silver nitrate, as a precursor, has been abundantly used due to its cheap price and easy availability [36–39]. Various reducing agents such as sodium citrate, sodium borohydride, and alcohols are widely used to reduce metal/Ag+ ions present in solution into metal/silver atoms which combine to make aggregates and finally become nanostructures. On the other hand, stabilizing agents are introduced to control and stabilize the required morphology of metal nanostructures [40–42]. However, in photochemical synthesis, a variety of light irradiation methods are adopted to synthesize metal nanostructures [43, 44]. Following is the detail of chemical/photochemical synthetic processes [45]. Various methods are used to produce metal nanostructures, which are summarized in Figure 1.Figure 1
Different methods of synthesis of metal nanostructures [1].


### 1.1. Light-Mediated Synthesis

In this technique, light irradiation is applied to produce nanostructures. An example is laser ablation or direct laser irradiation on an aqueous solution of metal salt in the presence of surfactants to fabricate specific shape and size distribution of metal nanoparticles where the source of light works as a reducing agent. Laser light is also used to modify metal nanoparticles by simple melting such as silver nanospheres into silver nanoplates, and it is known as the tailoring process with light. Laser light-mediated synthesis is continuing its success for generating highly desirable and well-controlled metal nanostructures [46, 47].


### 1.2. Electrolysis and Pyrolysis

There are few scientific reports available describing use of the electrochemical approach for the synthesis of metal nanoparticles [48–50]. For example, spherical-shaped silver nanoparticles were synthesized with an average size of 10 nm by reducing silver ions in the presence of poly (vinyl pyrrolidone) (PVP) using the electrochemical method, where titanium (Ti) electrode worked as a cathode and platinum (Pt) plate with 2 cm diameter worked as an anode [51–53]. Silver nanoparticles are also synthesized by adopting another method of spray pyrolysis in which an average grain size of 100 nm of silver nanopowder was synthesized [54–56]. Among other chemical syntheses, electrolysis and pyrolysis are considered environmental friendly processes as no harmful or toxic reducing agents are used to produce nanostructures [35].


### 1.3. Citrate Reduction

One of the popular methods to synthesize Ag colloidal solutions is citrate reduction of silver ions as we do not need comprehensive laboratory training skills [35]. First time, Lee and Meisel introduced this method in 1982 [181]. Generally, Ag nanoparticles are formed, when a calculated amount of sodium citrate is added in an aqueous solution of silver nitrate at boiling and kept for at least 1 h [57–59]. This is a very simple method but did not produce a confined size of nanoparticles. The synthesized product shows a variety of sizes in the range of 20–600 nm [60–62]. However, pH control is an important factor, as few shape-controlled methods had been discussed by previous researchers by controlling pH values such as at pH = 5.7 triangular shape and pH = 11.1 spherical as well as rod-like particles were formed [35].


### 1.4. Polyol Synthesis

Synthesized Ag nanostructures can be classified into different classes based on their properties, shape q s, or sizes [184]. In general, silver salt precursor and capping agents are introduced into polyols for the generation of nucleation and growth towards silver nanostructure [63–66]. Propylene glycol, 1,2-propylene glycol, or 1,5-pentanediol are commonly used reducing agents which reduce silver ions present in the solution [35]. Reaction parameters such as temperature and concentrations are very important to overcome the possible control on the final reaction product [67–70].


### 1.5. Seed-Mediated Synthesis

In recent years, another approach to synthesize metal nanoparticles gaining much attention is seed-mediated synthesis in which nanocrystals work as seeds for further growth [71–73]. This method has advantages on the control over the end product morphology. For example, Xia et al., in 2010, synthesized silver nanotubes by using the spherical- or cubic-shaped single-crystal seed for specific edge length from 30 to 200 nm [74–77].


### 1.6. Silver Mirror Reaction

Justus von Liebig discovered a silver mirror reaction in 1835 which became much popular for depositing Ag metal on targets or solid surfaces. In this reaction, Tollen’s reagent [Ag(NH3)2OH] is reduced by sugar or any aldehyde-containing compound for generating silver:(1)RCHO+2AgNH32++2OH−®RCOOH+2Ag+4NH3+H2OThe successful reaction shows shiny layer formation on the inner side of the reaction container due to the silver deposition. Silver mirror reaction is mostly employed for detecting the aldehyde group in schools.


## 1.1. Light-Mediated Synthesis

In this technique, light irradiation is applied to produce nanostructures. An example is laser ablation or direct laser irradiation on an aqueous solution of metal salt in the presence of surfactants to fabricate specific shape and size distribution of metal nanoparticles where the source of light works as a reducing agent. Laser light is also used to modify metal nanoparticles by simple melting such as silver nanospheres into silver nanoplates, and it is known as the tailoring process with light. Laser light-mediated synthesis is continuing its success for generating highly desirable and well-controlled metal nanostructures [46, 47].


## 1.2. Electrolysis and Pyrolysis

There are few scientific reports available describing use of the electrochemical approach for the synthesis of metal nanoparticles [48–50]. For example, spherical-shaped silver nanoparticles were synthesized with an average size of 10 nm by reducing silver ions in the presence of poly (vinyl pyrrolidone) (PVP) using the electrochemical method, where titanium (Ti) electrode worked as a cathode and platinum (Pt) plate with 2 cm diameter worked as an anode [51–53]. Silver nanoparticles are also synthesized by adopting another method of spray pyrolysis in which an average grain size of 100 nm of silver nanopowder was synthesized [54–56]. Among other chemical syntheses, electrolysis and pyrolysis are considered environmental friendly processes as no harmful or toxic reducing agents are used to produce nanostructures [35].


## 1.3. Citrate Reduction

One of the popular methods to synthesize Ag colloidal solutions is citrate reduction of silver ions as we do not need comprehensive laboratory training skills [35]. First time, Lee and Meisel introduced this method in 1982 [181]. Generally, Ag nanoparticles are formed, when a calculated amount of sodium citrate is added in an aqueous solution of silver nitrate at boiling and kept for at least 1 h [57–59]. This is a very simple method but did not produce a confined size of nanoparticles. The synthesized product shows a variety of sizes in the range of 20–600 nm [60–62]. However, pH control is an important factor, as few shape-controlled methods had been discussed by previous researchers by controlling pH values such as at pH = 5.7 triangular shape and pH = 11.1 spherical as well as rod-like particles were formed [35].


## 1.4. Polyol Synthesis

Synthesized Ag nanostructures can be classified into different classes based on their properties, shape q s, or sizes [184]. In general, silver salt precursor and capping agents are introduced into polyols for the generation of nucleation and growth towards silver nanostructure [63–66]. Propylene glycol, 1,2-propylene glycol, or 1,5-pentanediol are commonly used reducing agents which reduce silver ions present in the solution [35]. Reaction parameters such as temperature and concentrations are very important to overcome the possible control on the final reaction product [67–70].


## 1.5. Seed-Mediated Synthesis

In recent years, another approach to synthesize metal nanoparticles gaining much attention is seed-mediated synthesis in which nanocrystals work as seeds for further growth [71–73]. This method has advantages on the control over the end product morphology. For example, Xia et al., in 2010, synthesized silver nanotubes by using the spherical- or cubic-shaped single-crystal seed for specific edge length from 30 to 200 nm [74–77].


## 1.6. Silver Mirror Reaction

Justus von Liebig discovered a silver mirror reaction in 1835 which became much popular for depositing Ag metal on targets or solid surfaces. In this reaction, Tollen’s reagent [Ag(NH3)2OH] is reduced by sugar or any aldehyde-containing compound for generating silver:(1)RCHO+2AgNH32++2OH−®RCOOH+2Ag+4NH3+H2OThe successful reaction shows shiny layer formation on the inner side of the reaction container due to the silver deposition. Silver mirror reaction is mostly employed for detecting the aldehyde group in schools.


## 2. Synthesis of Metal Nanostructures with Controlled Morphology

### 2.1. Hard Template

Hard mold is used to produce metal nanowires with unique magnetic properties. In this method, the porous mold acts as a scaffold, and various materials are placed inside its pores and form wired nanostructures [78–80]. The hard mold contains inorganic porous materials such as polymer membranes, anodic aluminum oxide, and carbon nanotubes [9, 10]. The most commonly used hard molds are thin and porous anodic oxide films with various morphologies on aluminum sheets. Anodic aluminum oxide membranes are produced by the process of anodizing aluminum foil using acidic solutions [81, 82]. In anodic oxidation technology, the dimensions and distribution of the pores in the oxide thin film can be controlled [83]. Anodic aluminum oxide porous membranes with regular hexagonal pores have higher chemical and thermal stability and are therefore commonly used as a hard mold for the synthesis of metal nanowires [84, 85]. Also, the porosity density in these structures reaches 1011 holes per square centimeter [12, 13]. The use of the electrochemical deposition method to fill the desired material inside porous membranes is a simple and flexible approach to the synthesis of one-dimensional nanostructures that have been used to produce various types of metal and semiconductor nanowires [86, 87]. In general, the synthesis of nanomaterials inside hard mold spores has three main steps as follows [12–14]:(A)
Coating one side of the membrane using a thin metal layer by electrochemical deposition and using it as a plating electrode [88](B)
Catholic deposition of ions of the desired material from inside the solution on the metal surface in the pore floor [89](C)
Dissolution of anodic oxide molds in acid or base and obtaining separate metal nanowires [90]A schematic of a common hard mold consisting of anodic aluminum oxide membranes is shown in Figure2(a) [2]. One-dimensional channels with controllable length and diameter are filled with the desired materials, and then, regular nanowires are obtained by dissolving the mold in a solution of one molar NaOH [91, 92]. Figure 2(b) shows a scanning electron microscope image of anodic aluminum oxide molds with a pore diameter of 60 nm that can be dissolved in sulfuric acid solution [2]. The length and diameter of the pores as well as the distance between them in the mold can be controlled by changing the applied voltage and the electrolyte used in the anodizing process. Scanning electron microscope image of metal nanowires produced by hard molding method is shown in Figure 2(c) [2]. The synthesized nanowires are continuous and approximately parallel to a diameter of approximately 50 nm, slightly smaller than the diameter of the mold spores [93, 94]. This is because nanowires are compacted by removing water from their structure [95]. The higher the growth rate of nanowires, the more structural defects occur in nanowires [96]. Unlike soft mold synthesis, hard molded metal nanowires usually have a polycrystalline or polycrystalline structure. Since the magnetic nature of nanomaterials strongly depends on the degree of crystallinity of the structure, subsequent annealing heat treatment is usually used to improve the magnetic properties of nanowires produced by hard molding [97, 98]. Here, the porous alumina mold acts as a protective mold during the annealing operation of nanowires due to its high chemical and thermal stability [99, 100]. A diagram of the synthesis of CoPt3 nanowires during the deposition process of cobalt and platinum atomic clusters is shown in Figure 3 on a thin film of amorphous or noncrystalline gold on the surface of the anodic oxide layer. In the early stages of CoPt3 growth, the buds are randomly oriented, and the atomic mass is deposited continuously in a completely random direction [101–103]. During the cobalt and platinum precipitation process, CoPt3 nanowires with lower crystallinity are formed with some porosity and internal stress [104]. Stress anisotropy directly leads to dimensional anisotropy and reduces magnetic coercively [105, 106]. Therefore, the magnetic fields (indicated by arrows) will have a random and small orientation [15–18]. Heat treatment increases magnetic induction during structural failure or stress drop and reduces the number of defects [107–109]. Also, the magneto static vector is preferably positioned along the axis of the wire to reduce the magneto static energy [110, 111]. Therefore, the direction of magnetization of nanometer arrays is in the direction of the axis of the nanowires and causes the magnetic inhibitory force (HC) of the parallel field to be greater than the HC of the field perpendicular [112–114]. By heating or so-called annealing of CoPt3 nanowires at 400°C, the nanowire structure will contain cobalt ferromagnetic nanoclusters in clusters [115, 116]. Platinum atoms separate cobalt atomic clusters and facilitate the formation of cobalt nanoclusters with a single magnetic field [117, 118]. By increasing the annealing temperature to 500°C, CoPt3 nanowires maintain their cluster structure. Cobalt and platinum atoms begin to atomically migrate through the diffusion mechanism to form a regular structure (L12) [119, 120]. This creates a strong ferromagnetic coupling between the Co and Pt atoms by hybridizing the 3d cobalt and 5d Pt orbitals, and the spin polarization of the Pt atoms [121]. Co clusters (blue arrows) and the regular structure of CoPt3 (yellow arrows) both play a role in improving the magnetic properties (increasing the HC triplet) of nanowires [122]. Finally, as the temperature rises to 700°C, most CoPt3 nanowires form a regular, long-range structure. Reducing the density of defects and the number of grains in the growing CoPt3 leads to a significant increase in magnetic induction [19, 20]. The hard die-casting process has been widely considered due to its high controllability in the precise production of one-dimensional nanomaterials, especially magnetic nanowires [123–125]. Also, significant progress has been made in the production of intermediate metal nanowires such as iron, cobalt, nickel, and their alloys using porous alumina molds [126, 127].Figure 2
(a) Scheme of hard mold, (b) the scanning electron microscope image of anodic aluminum oxide mold, and (c) the scanning electron microscope image of CoPt3 nanowires produced by the hard mold method [2].
(a)(b)(c)Figure 3
Schematic of the phase conversion steps of CoPt3 nanowires [26].


### 2.2. Soft Template

Soft molds refer to a set of surfactants or surfactants such as liquid crystals, micelles. Unlike hard molds, soft molds are of organic origin and contain ligands, surfactants, and polymers [128–131]. Using soft molds such as rod-shaped micelles, Ag and gold nanowires and nanowires with controllable diameters and dimensional ratios can be synthesized. Bar-shaped micelles are obtained by self-assembly of cetyltrimethylammonium bromide (CTAB) or liquid crystalline phases consisting of sodium bis (2-ethylhexyl) sulfosuccinate, para-xylene, and water [21–25]. Also, the two-layer acetyl amine/water system is used as a soft mold for the production of Ag nanosheets. Compared to the hard mold method, the rate of germination and growth reactions is slower in the soft mold method [132–134]. As a result, the rate of germination and growth is practically controllable, and distinct crystalline faces are clearly visible on the surface of the nanoparticles [135, 136]. The morphology of metal nanostructures is highly sensitive to the temperature of the growth solution used in the soft molding method [26]. Ag diagram of the formation of gold nanostructures with different morphologies at different temperatures, along with transmission electron microscope images of each of them, is shown in Figure 4. At high temperatures, rapid reduction of -AuCl4 ions provides gold buds, and CTAB interactions with these particles at high temperatures contribute to the growth of gold nanoparticles [137–139]. By changing the temperature, the growth kinetics of gold nanostructures can be controlled to produce nanosheets, icosahedral nanocrystals, spherical and hexagonal nanoparticles, and gold nanowires [28, 140, 141].Figure 4
Scheme of gold nanostructures with different morphologies formed at different temperatures along with transmission electron microscope images related to them [28].


### 2.3. Sacrificial Template

In recent years, the synthesis and characterization of hollow nanomaterials due to its potential application have attracted much attention in the development of nanoscience and nanotechnology. Various methods have been developed for the synthesis of hollow inorganic nanomaterials, including the Ostwald processing process, the crackling effect, and the sacrificial mold [142–146]. The sacrificial mold method is an effective method for the preparation of hollow metal nanostructures [147]. The most important reaction in this method is the reaction between a reactive sacrificial mold and a reactant (neutral component). The reaction between these two compounds occurs at the mold surface [148]. The neutral component deposits on the surface of the mold and penetrates out of the pores. The morphology of the obtained hollow nanostructures is highly dependent on the shape of the sacrificial mold. Gold and silver are widely used in sacrificial molding reactions due to their special optical properties [149]. When a sacrificial mold of silver and gold tetrachloride (AuCl-4) is mixed in an aqueous solution, because the standard potential of the -Au/AuCl4 pair (0.99 Volts relative to the standard hydrogen electrode) is greater than the standard potential of the silver/silver pair (0.8 Volts relative to the standard hydrogen electrode), the silver atoms are oxidized to Ag + Ag ions [150, 151]. The galvanic substitution process is used to synthesize hollow gold nanospheres. In the first step, a layer of pure silver is deposited on the surface of the gold nanoparticles [29, 30]. By depositing silver atoms directly on the surface of Au nanoparticles using an ascorbic acid reducing agent, gold-silver core-shell nanoparticles are obtained. The proximity of the atomic radii of silver and gold causes the germination and growth of silver atomic clusters on the surface of gold nanoparticles (Figure 5(a)). The second step is the galvanic substitution reaction between the shell and core colloids with aqueous HAuCl4 solution at room temperature. The chemical reaction is as follows [31–33]:(2)3Ag+AuCl−4 3Ag++4Cl−+AuFigure 5
Schematic of the synthesis of hollow gold nanospheres during galvanic substitution with associated transmission electron microscope images [29].
(a)(b)(c)(d)Figures5(b) and 5(c) show gold nanoparticles, Aucore Agshell core-shell nanoparticles, and hollow gold nanoparticles, respectively. When silver nanoparticles are coated with a solution of gold ions, the pure layers of silver turn into gold nanoshells [29].


## 2.1. Hard Template

Hard mold is used to produce metal nanowires with unique magnetic properties. In this method, the porous mold acts as a scaffold, and various materials are placed inside its pores and form wired nanostructures [78–80]. The hard mold contains inorganic porous materials such as polymer membranes, anodic aluminum oxide, and carbon nanotubes [9, 10]. The most commonly used hard molds are thin and porous anodic oxide films with various morphologies on aluminum sheets. Anodic aluminum oxide membranes are produced by the process of anodizing aluminum foil using acidic solutions [81, 82]. In anodic oxidation technology, the dimensions and distribution of the pores in the oxide thin film can be controlled [83]. Anodic aluminum oxide porous membranes with regular hexagonal pores have higher chemical and thermal stability and are therefore commonly used as a hard mold for the synthesis of metal nanowires [84, 85]. Also, the porosity density in these structures reaches 1011 holes per square centimeter [12, 13]. The use of the electrochemical deposition method to fill the desired material inside porous membranes is a simple and flexible approach to the synthesis of one-dimensional nanostructures that have been used to produce various types of metal and semiconductor nanowires [86, 87]. In general, the synthesis of nanomaterials inside hard mold spores has three main steps as follows [12–14]:(A)
Coating one side of the membrane using a thin metal layer by electrochemical deposition and using it as a plating electrode [88](B)
Catholic deposition of ions of the desired material from inside the solution on the metal surface in the pore floor [89](C)
Dissolution of anodic oxide molds in acid or base and obtaining separate metal nanowires [90]A schematic of a common hard mold consisting of anodic aluminum oxide membranes is shown in Figure2(a) [2]. One-dimensional channels with controllable length and diameter are filled with the desired materials, and then, regular nanowires are obtained by dissolving the mold in a solution of one molar NaOH [91, 92]. Figure 2(b) shows a scanning electron microscope image of anodic aluminum oxide molds with a pore diameter of 60 nm that can be dissolved in sulfuric acid solution [2]. The length and diameter of the pores as well as the distance between them in the mold can be controlled by changing the applied voltage and the electrolyte used in the anodizing process. Scanning electron microscope image of metal nanowires produced by hard molding method is shown in Figure 2(c) [2]. The synthesized nanowires are continuous and approximately parallel to a diameter of approximately 50 nm, slightly smaller than the diameter of the mold spores [93, 94]. This is because nanowires are compacted by removing water from their structure [95]. The higher the growth rate of nanowires, the more structural defects occur in nanowires [96]. Unlike soft mold synthesis, hard molded metal nanowires usually have a polycrystalline or polycrystalline structure. Since the magnetic nature of nanomaterials strongly depends on the degree of crystallinity of the structure, subsequent annealing heat treatment is usually used to improve the magnetic properties of nanowires produced by hard molding [97, 98]. Here, the porous alumina mold acts as a protective mold during the annealing operation of nanowires due to its high chemical and thermal stability [99, 100]. A diagram of the synthesis of CoPt3 nanowires during the deposition process of cobalt and platinum atomic clusters is shown in Figure 3 on a thin film of amorphous or noncrystalline gold on the surface of the anodic oxide layer. In the early stages of CoPt3 growth, the buds are randomly oriented, and the atomic mass is deposited continuously in a completely random direction [101–103]. During the cobalt and platinum precipitation process, CoPt3 nanowires with lower crystallinity are formed with some porosity and internal stress [104]. Stress anisotropy directly leads to dimensional anisotropy and reduces magnetic coercively [105, 106]. Therefore, the magnetic fields (indicated by arrows) will have a random and small orientation [15–18]. Heat treatment increases magnetic induction during structural failure or stress drop and reduces the number of defects [107–109]. Also, the magneto static vector is preferably positioned along the axis of the wire to reduce the magneto static energy [110, 111]. Therefore, the direction of magnetization of nanometer arrays is in the direction of the axis of the nanowires and causes the magnetic inhibitory force (HC) of the parallel field to be greater than the HC of the field perpendicular [112–114]. By heating or so-called annealing of CoPt3 nanowires at 400°C, the nanowire structure will contain cobalt ferromagnetic nanoclusters in clusters [115, 116]. Platinum atoms separate cobalt atomic clusters and facilitate the formation of cobalt nanoclusters with a single magnetic field [117, 118]. By increasing the annealing temperature to 500°C, CoPt3 nanowires maintain their cluster structure. Cobalt and platinum atoms begin to atomically migrate through the diffusion mechanism to form a regular structure (L12) [119, 120]. This creates a strong ferromagnetic coupling between the Co and Pt atoms by hybridizing the 3d cobalt and 5d Pt orbitals, and the spin polarization of the Pt atoms [121]. Co clusters (blue arrows) and the regular structure of CoPt3 (yellow arrows) both play a role in improving the magnetic properties (increasing the HC triplet) of nanowires [122]. Finally, as the temperature rises to 700°C, most CoPt3 nanowires form a regular, long-range structure. Reducing the density of defects and the number of grains in the growing CoPt3 leads to a significant increase in magnetic induction [19, 20]. The hard die-casting process has been widely considered due to its high controllability in the precise production of one-dimensional nanomaterials, especially magnetic nanowires [123–125]. Also, significant progress has been made in the production of intermediate metal nanowires such as iron, cobalt, nickel, and their alloys using porous alumina molds [126, 127].Figure 2
(a) Scheme of hard mold, (b) the scanning electron microscope image of anodic aluminum oxide mold, and (c) the scanning electron microscope image of CoPt3 nanowires produced by the hard mold method [2].
(a)(b)(c)Figure 3
Schematic of the phase conversion steps of CoPt3 nanowires [26].


## 2.2. Soft Template

Soft molds refer to a set of surfactants or surfactants such as liquid crystals, micelles. Unlike hard molds, soft molds are of organic origin and contain ligands, surfactants, and polymers [128–131]. Using soft molds such as rod-shaped micelles, Ag and gold nanowires and nanowires with controllable diameters and dimensional ratios can be synthesized. Bar-shaped micelles are obtained by self-assembly of cetyltrimethylammonium bromide (CTAB) or liquid crystalline phases consisting of sodium bis (2-ethylhexyl) sulfosuccinate, para-xylene, and water [21–25]. Also, the two-layer acetyl amine/water system is used as a soft mold for the production of Ag nanosheets. Compared to the hard mold method, the rate of germination and growth reactions is slower in the soft mold method [132–134]. As a result, the rate of germination and growth is practically controllable, and distinct crystalline faces are clearly visible on the surface of the nanoparticles [135, 136]. The morphology of metal nanostructures is highly sensitive to the temperature of the growth solution used in the soft molding method [26]. Ag diagram of the formation of gold nanostructures with different morphologies at different temperatures, along with transmission electron microscope images of each of them, is shown in Figure 4. At high temperatures, rapid reduction of -AuCl4 ions provides gold buds, and CTAB interactions with these particles at high temperatures contribute to the growth of gold nanoparticles [137–139]. By changing the temperature, the growth kinetics of gold nanostructures can be controlled to produce nanosheets, icosahedral nanocrystals, spherical and hexagonal nanoparticles, and gold nanowires [28, 140, 141].Figure 4
Scheme of gold nanostructures with different morphologies formed at different temperatures along with transmission electron microscope images related to them [28].


## 2.3. Sacrificial Template

In recent years, the synthesis and characterization of hollow nanomaterials due to its potential application have attracted much attention in the development of nanoscience and nanotechnology. Various methods have been developed for the synthesis of hollow inorganic nanomaterials, including the Ostwald processing process, the crackling effect, and the sacrificial mold [142–146]. The sacrificial mold method is an effective method for the preparation of hollow metal nanostructures [147]. The most important reaction in this method is the reaction between a reactive sacrificial mold and a reactant (neutral component). The reaction between these two compounds occurs at the mold surface [148]. The neutral component deposits on the surface of the mold and penetrates out of the pores. The morphology of the obtained hollow nanostructures is highly dependent on the shape of the sacrificial mold. Gold and silver are widely used in sacrificial molding reactions due to their special optical properties [149]. When a sacrificial mold of silver and gold tetrachloride (AuCl-4) is mixed in an aqueous solution, because the standard potential of the -Au/AuCl4 pair (0.99 Volts relative to the standard hydrogen electrode) is greater than the standard potential of the silver/silver pair (0.8 Volts relative to the standard hydrogen electrode), the silver atoms are oxidized to Ag + Ag ions [150, 151]. The galvanic substitution process is used to synthesize hollow gold nanospheres. In the first step, a layer of pure silver is deposited on the surface of the gold nanoparticles [29, 30]. By depositing silver atoms directly on the surface of Au nanoparticles using an ascorbic acid reducing agent, gold-silver core-shell nanoparticles are obtained. The proximity of the atomic radii of silver and gold causes the germination and growth of silver atomic clusters on the surface of gold nanoparticles (Figure 5(a)). The second step is the galvanic substitution reaction between the shell and core colloids with aqueous HAuCl4 solution at room temperature. The chemical reaction is as follows [31–33]:(2)3Ag+AuCl−4 3Ag++4Cl−+AuFigure 5
Schematic of the synthesis of hollow gold nanospheres during galvanic substitution with associated transmission electron microscope images [29].
(a)(b)(c)(d)Figures5(b) and 5(c) show gold nanoparticles, Aucore Agshell core-shell nanoparticles, and hollow gold nanoparticles, respectively. When silver nanoparticles are coated with a solution of gold ions, the pure layers of silver turn into gold nanoshells [29].


## 3. Properties and Applications of Metal Nanoparticles

### 3.1. Optical Properties and Applications

As mentioned, free electrons in metals are stimulated by light and oscillate collectively, which is called the superficial plasmon resonance process [152, 153]. This mass oscillation can be effective in scattering and absorbing light in conditions of intensification and bright colors of metals, especially gold and silver [154, 155]. When stimulated by light on a metal surface, the oscillation of the electrons causes the surface charges to polarize. Unlike the bulk mode, electric charges cannot be distributed on the surface of light-excited nanoparticles due to their short wavelength, which is called localized surface plasmon resonance (LSPR) [34, 36]. This phenomenon creates strong localized electric fields at the nanometer scale [156, 157]. LSPR is used in the Raman scattering spectra of molecules and the detection of fingerprint spectra using chemical information. Due to their plasmatic behavior, gold and silver nanocrystals are used in Raman-enhanced surface dispersion spectroscopy (SERS) and molecular sensors [158–160]. Although each organic molecule has a specific Raman spectrum, signal amplification is important to detect low-concentration analyses [161, 162]. To this end, the Raman signal of adsorbed molecules on the surface can be amplified by applying strong local electric fields by LSPR and metal nanocrystals [37].


### 3.2. Catalytic Properties and Applications

The use of metal nanoparticles is very beneficial to facilitate chemical reactions. These materials have good catalytic performance due to the high surface-to-volume ratio [163], [164, 165]. These metal nanocrystals with engineered structures are used as catalysts for oxidation reactions, carbon-carbon coupling, electron transfer, hydrogenation, etc. Even certain metal nanostructures have been used in industrial-scale catalytic applications [38]. It is worth noting that the reactivity and selectivity of this type of catalyst can be controlled by the morphology of nanocrystals [35]. In addition to the total surface area of the crystal, the morphology of the crystal also determines the number of atoms at the edges and corners of the crystal [35]. These factors affect their catalytic performance. Optimization of these factors should be considered as a criterion for evaluating the performance of nanocrystal line catalysts [35] which is also necessary to pay attention to the stability of nanoparticles and the factors covering the surface of nanocatalysts. In many cases, the atoms on the surface of nanocatalysts are very active and change their shape, dimensions, and lifespan during the catalytic reaction [35, 160–163]. Also, particle surface coating agents may inactivate catalytically active regions [35, 160–163].


### 3.3. Properties and Applications of Electronics

Some metal nanocrystals, such as silver nanowires and silver nanorods, have electronic applications due to their high thermal and electrical conductivity [35]. Measurement of resistance and electrical conductivity of silver nanorods with different lengths and diameters indicates that the conductivity of silver nanorods with a diameter of 20 nm is twice that of bulk conductors, and despite their very small size, these nanorods have extremely high electrical conductivity compared to bulk samples [35, 162, 163]. Therefore, metal nanocrystals with unique electronic properties are a very good choice for use in the manufacture of electrodes [39, 40].


### 3.4. Properties and Applications of Magnetism

Changing the dimensions and morphology of metal nanocrystals has a great impact on its fundamental magnetic properties [158]. As the size of nanocrystals decreases, new magnetic properties are observed. The property of superparamagnetic is one of the most important phenomena related to the size of magnetic nanoparticles [159–162]. The anisotropic magnetic energy barrier from the upward spin to the downward spin state for bulk materials is much larger than the thermal energy (kT). The anisotropic magnetic energy barrier of a magnet is proportional to the product of a constant multiplication of magnetic anisotropy (Ku) in the volume of the magnet [163]. In the case of nanoparticles, however, the thermal energy of the nanoparticles is easily sufficient to reverse the direction of the magnetic spin, although it is not sufficient to overcome the spin-spin exchange coupling energy [35]. This behavior, in which the magnetic oscillation results in a total magnetization of zero, is called the superparamagnetic property, and the temperature at which the ferromagnetic to superparamagnetic conversion occurs is known as the blocking temperature (Tb) and is defined using the following relation [41, 42].In this regard, V is the volume of the nanocrystal,k is the temperature in Kelvin, and Tb is the critical temperature for converting ferrimagnets to super paramagnets, or the blocking temperature.(3)Tb=KuV25K.


## 3.1. Optical Properties and Applications

As mentioned, free electrons in metals are stimulated by light and oscillate collectively, which is called the superficial plasmon resonance process [152, 153]. This mass oscillation can be effective in scattering and absorbing light in conditions of intensification and bright colors of metals, especially gold and silver [154, 155]. When stimulated by light on a metal surface, the oscillation of the electrons causes the surface charges to polarize. Unlike the bulk mode, electric charges cannot be distributed on the surface of light-excited nanoparticles due to their short wavelength, which is called localized surface plasmon resonance (LSPR) [34, 36]. This phenomenon creates strong localized electric fields at the nanometer scale [156, 157]. LSPR is used in the Raman scattering spectra of molecules and the detection of fingerprint spectra using chemical information. Due to their plasmatic behavior, gold and silver nanocrystals are used in Raman-enhanced surface dispersion spectroscopy (SERS) and molecular sensors [158–160]. Although each organic molecule has a specific Raman spectrum, signal amplification is important to detect low-concentration analyses [161, 162]. To this end, the Raman signal of adsorbed molecules on the surface can be amplified by applying strong local electric fields by LSPR and metal nanocrystals [37].


## 3.2. Catalytic Properties and Applications

The use of metal nanoparticles is very beneficial to facilitate chemical reactions. These materials have good catalytic performance due to the high surface-to-volume ratio [163], [164, 165]. These metal nanocrystals with engineered structures are used as catalysts for oxidation reactions, carbon-carbon coupling, electron transfer, hydrogenation, etc. Even certain metal nanostructures have been used in industrial-scale catalytic applications [38]. It is worth noting that the reactivity and selectivity of this type of catalyst can be controlled by the morphology of nanocrystals [35]. In addition to the total surface area of the crystal, the morphology of the crystal also determines the number of atoms at the edges and corners of the crystal [35]. These factors affect their catalytic performance. Optimization of these factors should be considered as a criterion for evaluating the performance of nanocrystal line catalysts [35] which is also necessary to pay attention to the stability of nanoparticles and the factors covering the surface of nanocatalysts. In many cases, the atoms on the surface of nanocatalysts are very active and change their shape, dimensions, and lifespan during the catalytic reaction [35, 160–163]. Also, particle surface coating agents may inactivate catalytically active regions [35, 160–163].


## 3.3. Properties and Applications of Electronics

Some metal nanocrystals, such as silver nanowires and silver nanorods, have electronic applications due to their high thermal and electrical conductivity [35]. Measurement of resistance and electrical conductivity of silver nanorods with different lengths and diameters indicates that the conductivity of silver nanorods with a diameter of 20 nm is twice that of bulk conductors, and despite their very small size, these nanorods have extremely high electrical conductivity compared to bulk samples [35, 162, 163]. Therefore, metal nanocrystals with unique electronic properties are a very good choice for use in the manufacture of electrodes [39, 40].


## 3.4. Properties and Applications of Magnetism

Changing the dimensions and morphology of metal nanocrystals has a great impact on its fundamental magnetic properties [158]. As the size of nanocrystals decreases, new magnetic properties are observed. The property of superparamagnetic is one of the most important phenomena related to the size of magnetic nanoparticles [159–162]. The anisotropic magnetic energy barrier from the upward spin to the downward spin state for bulk materials is much larger than the thermal energy (kT). The anisotropic magnetic energy barrier of a magnet is proportional to the product of a constant multiplication of magnetic anisotropy (Ku) in the volume of the magnet [163]. In the case of nanoparticles, however, the thermal energy of the nanoparticles is easily sufficient to reverse the direction of the magnetic spin, although it is not sufficient to overcome the spin-spin exchange coupling energy [35]. This behavior, in which the magnetic oscillation results in a total magnetization of zero, is called the superparamagnetic property, and the temperature at which the ferromagnetic to superparamagnetic conversion occurs is known as the blocking temperature (Tb) and is defined using the following relation [41, 42].In this regard, V is the volume of the nanocrystal,k is the temperature in Kelvin, and Tb is the critical temperature for converting ferrimagnets to super paramagnets, or the blocking temperature.(3)Tb=KuV25K.


## 4. Nobel Metal Nanomaterials

### 4.1. Nanoelectrocatalysts Based on Noble Metal Nanostructures for Fuel Cell Applications

The development of nanomaterial synthesis methods has led to the production of various metal nanostructures with different dimensions, morphology, chemical composition, and antiparticle interactions [146]. These nanostructures provide a good opportunity to develop the application of active nanocrystals in fuel cell reactions. Platinum and platinum-based nanostructures are still the most effective electrocatalysts for fuel cell applications [147]. Small spherical nanoparticles are used as electrocatalysts in the oxidation reaction of small molecules because they have a high surface-to-volume ratio [149]. By reducing the dimensions of nanoparticles, their active surface increases, and therefore, in the structure of commercial electrocatalysts, platinum and palladium nanoparticles with very small dimensions (3 nm) are used [159]. It has also recently been shown that reducing the size of platinum nanoparticles plays a key role in increasing their catalytic activity. For example, platinum nanotubes synthesized using silver nanoparticles as a sacrificial mold have electrochemical activity and a longer lifespan than commercial catalysts [43].


### 4.2. Noble Metal Nanomaterials for Analytical Sensors

#### 4.2.1. Noble Metal Nanomaterials for Electrochemical Sensors

Advances in nanoscience have opened up new horizons in the field of electrochemistry [35]. The use of nanomaterials to make electrochemical sensors has increased their sensitivity and selectivity and has made it possible to detect target molecules based on various statistical analysis methods [160]. Improving the electrochemical sensitivity of these sensors requires the use of better electrode nanomaterials such as gold nanoparticles and mineral nanomaterials for electroanalytical applications. Because nanomaterials are able to increase the electrochemically active surface to absorb target molecules and facilitate electron transfer between the electrode and molecules and therefore create a higher response speed and higher sensitivity [42].


#### 4.2.2. Noble Metal Nanomaterials for Calorimetry Sensors

Colorimetric sensors are attractive because of their simplicity, high sensitivity, and low price. These sensors can be coupled with UV-Vis spectroscopy to replace complex devices. The solution of gold nanoparticles is red but turns purple or blue after changing color in colorimetric sensors [116]. Therefore, at present, the color change of a solution of gold nanoparticles in colorimetric sensors is used to identify DNA, enzymatic activity, small molecules, metal ions, and proteins. Gold nanoparticles in combination with other molecules can provide a good sensor for detecting target molecules [119]. In optical sensors, silver nanoparticles are more useful because they have a higher absorption coefficient than gold nanoparticles with the same dimensions. Thus, the combination of silver nanoparticles with DNA molecules and other molecules results in the formation of a biodegradable colorimetric biosensor that is used to detect some important analyses. Unlike Au and Ag nanoparticles, Pt and Pd nanoparticles in the visible absorption spectrum do not have surface plasmon resonance (SPR) and therefore are not used in colorimetric sensors [120]. Figure 6 modifies a sensitive, simple colorimetric sensor based on aptameric (Aptameric, oligonucleotides synthesized from RNA or DNA or a combination of the two with other molecules) to detect thrombin (Thrombin, an enzyme needed for blood clotting) using gold nanoparticles. When thrombin is added to a solution of modified gold nanoparticles, thrombin interacts with aptameric on the surface of gold nanoparticles, and after adding a high concentration of sodium chloride salt, the color changes of the gold nanoparticles will be able to detect small amounts of thrombin [41–44].Figure 6
Gold nanoparticle colorimetric process for thrombin detection [30].


#### 4.2.3. Noble Metal Nanomaterials for Fluorescence Sensors

Fluorescence sensors are attractive because of their higher sensitivity and better performance than other optical methods. Recent advances in the field of synthesis and characterization of new metal nanomaterials have provided a good basis for the design of biochemical and fluorescence sensors [158]. In general, the design of fluorescence sensors based on metal nanomaterials is done in the following four ways [35, 162, 163]:(A)
The use of fluorescence intensity reduction or “fluorescence quenching” processes of metal nanoparticles by target material molecules: various types of molecular interactions, including excited state reactions, molecular rearrangements, energy transfer, base state complex formation, and collisional or dynamic extinction, can lead to fluorescence extinction. For example, silver nanoparticles are used to detect mercury ions (Hg+2) with low detection limits and high selectivity (Figure 7) [31].(B)
Fluorescence quenching of metal nanoparticles by electron/nonradiative energy transfer mechanism: for example, gold nanoprobes are used to simultaneously identify three analyses of combined adenosine (A), potassium ion (K+), and cocaine, using multiple dyes (Figure 8) [31].(C)
Inner filter effect (IFE) fluorescence based on metal nanoparticles: here, metal nanoparticles act as fluorescence adsorbents for fluorophore emission units (fluorescence generators). It has been shown that gold nanoparticles can be used as a strong adsorbent in IFE fluorescence to detect cyanide (−CN) and hydrogen peroxide (H2O2) (Figure 9) [42].(D)
Metal-enhanced fluorescence (MEF): this means that the emission of fluorophore at a certain distance from metal nanostructures (5–10 nm) can increase. This is an interesting sensor for increasing the detection limit of target molecules (Figure10) [41–44].Figure 7
Fluorescence extinction mechanism for high-sensitivity Hg2 + detection [31].Figure 8
Multicolored gold nanoprobe for simultaneous detection of adenosine, potassium, and cocaine [42].Figure 9
Scheme of IFE-based fluorescence detection process for hydrogen cyanide and hydrogen peroxide [43].
(a)(b)Figure 10
Scheme of metal-enhanced fluorescence (MEF) [44].


## 4.1. Nanoelectrocatalysts Based on Noble Metal Nanostructures for Fuel Cell Applications

The development of nanomaterial synthesis methods has led to the production of various metal nanostructures with different dimensions, morphology, chemical composition, and antiparticle interactions [146]. These nanostructures provide a good opportunity to develop the application of active nanocrystals in fuel cell reactions. Platinum and platinum-based nanostructures are still the most effective electrocatalysts for fuel cell applications [147]. Small spherical nanoparticles are used as electrocatalysts in the oxidation reaction of small molecules because they have a high surface-to-volume ratio [149]. By reducing the dimensions of nanoparticles, their active surface increases, and therefore, in the structure of commercial electrocatalysts, platinum and palladium nanoparticles with very small dimensions (3 nm) are used [159]. It has also recently been shown that reducing the size of platinum nanoparticles plays a key role in increasing their catalytic activity. For example, platinum nanotubes synthesized using silver nanoparticles as a sacrificial mold have electrochemical activity and a longer lifespan than commercial catalysts [43].


## 4.2. Noble Metal Nanomaterials for Analytical Sensors

### 4.2.1. Noble Metal Nanomaterials for Electrochemical Sensors

Advances in nanoscience have opened up new horizons in the field of electrochemistry [35]. The use of nanomaterials to make electrochemical sensors has increased their sensitivity and selectivity and has made it possible to detect target molecules based on various statistical analysis methods [160]. Improving the electrochemical sensitivity of these sensors requires the use of better electrode nanomaterials such as gold nanoparticles and mineral nanomaterials for electroanalytical applications. Because nanomaterials are able to increase the electrochemically active surface to absorb target molecules and facilitate electron transfer between the electrode and molecules and therefore create a higher response speed and higher sensitivity [42].


### 4.2.2. Noble Metal Nanomaterials for Calorimetry Sensors

Colorimetric sensors are attractive because of their simplicity, high sensitivity, and low price. These sensors can be coupled with UV-Vis spectroscopy to replace complex devices. The solution of gold nanoparticles is red but turns purple or blue after changing color in colorimetric sensors [116]. Therefore, at present, the color change of a solution of gold nanoparticles in colorimetric sensors is used to identify DNA, enzymatic activity, small molecules, metal ions, and proteins. Gold nanoparticles in combination with other molecules can provide a good sensor for detecting target molecules [119]. In optical sensors, silver nanoparticles are more useful because they have a higher absorption coefficient than gold nanoparticles with the same dimensions. Thus, the combination of silver nanoparticles with DNA molecules and other molecules results in the formation of a biodegradable colorimetric biosensor that is used to detect some important analyses. Unlike Au and Ag nanoparticles, Pt and Pd nanoparticles in the visible absorption spectrum do not have surface plasmon resonance (SPR) and therefore are not used in colorimetric sensors [120]. Figure 6 modifies a sensitive, simple colorimetric sensor based on aptameric (Aptameric, oligonucleotides synthesized from RNA or DNA or a combination of the two with other molecules) to detect thrombin (Thrombin, an enzyme needed for blood clotting) using gold nanoparticles. When thrombin is added to a solution of modified gold nanoparticles, thrombin interacts with aptameric on the surface of gold nanoparticles, and after adding a high concentration of sodium chloride salt, the color changes of the gold nanoparticles will be able to detect small amounts of thrombin [41–44].Figure 6
Gold nanoparticle colorimetric process for thrombin detection [30].


### 4.2.3. Noble Metal Nanomaterials for Fluorescence Sensors

Fluorescence sensors are attractive because of their higher sensitivity and better performance than other optical methods. Recent advances in the field of synthesis and characterization of new metal nanomaterials have provided a good basis for the design of biochemical and fluorescence sensors [158]. In general, the design of fluorescence sensors based on metal nanomaterials is done in the following four ways [35, 162, 163]:(A)
The use of fluorescence intensity reduction or “fluorescence quenching” processes of metal nanoparticles by target material molecules: various types of molecular interactions, including excited state reactions, molecular rearrangements, energy transfer, base state complex formation, and collisional or dynamic extinction, can lead to fluorescence extinction. For example, silver nanoparticles are used to detect mercury ions (Hg+2) with low detection limits and high selectivity (Figure 7) [31].(B)
Fluorescence quenching of metal nanoparticles by electron/nonradiative energy transfer mechanism: for example, gold nanoprobes are used to simultaneously identify three analyses of combined adenosine (A), potassium ion (K+), and cocaine, using multiple dyes (Figure 8) [31].(C)
Inner filter effect (IFE) fluorescence based on metal nanoparticles: here, metal nanoparticles act as fluorescence adsorbents for fluorophore emission units (fluorescence generators). It has been shown that gold nanoparticles can be used as a strong adsorbent in IFE fluorescence to detect cyanide (−CN) and hydrogen peroxide (H2O2) (Figure 9) [42].(D)
Metal-enhanced fluorescence (MEF): this means that the emission of fluorophore at a certain distance from metal nanostructures (5–10 nm) can increase. This is an interesting sensor for increasing the detection limit of target molecules (Figure10) [41–44].Figure 7
Fluorescence extinction mechanism for high-sensitivity Hg2 + detection [31].Figure 8
Multicolored gold nanoprobe for simultaneous detection of adenosine, potassium, and cocaine [42].Figure 9
Scheme of IFE-based fluorescence detection process for hydrogen cyanide and hydrogen peroxide [43].
(a)(b)Figure 10
Scheme of metal-enhanced fluorescence (MEF) [44].


## 4.2.1. Noble Metal Nanomaterials for Electrochemical Sensors

Advances in nanoscience have opened up new horizons in the field of electrochemistry [35]. The use of nanomaterials to make electrochemical sensors has increased their sensitivity and selectivity and has made it possible to detect target molecules based on various statistical analysis methods [160]. Improving the electrochemical sensitivity of these sensors requires the use of better electrode nanomaterials such as gold nanoparticles and mineral nanomaterials for electroanalytical applications. Because nanomaterials are able to increase the electrochemically active surface to absorb target molecules and facilitate electron transfer between the electrode and molecules and therefore create a higher response speed and higher sensitivity [42].


## 4.2.2. Noble Metal Nanomaterials for Calorimetry Sensors

Colorimetric sensors are attractive because of their simplicity, high sensitivity, and low price. These sensors can be coupled with UV-Vis spectroscopy to replace complex devices. The solution of gold nanoparticles is red but turns purple or blue after changing color in colorimetric sensors [116]. Therefore, at present, the color change of a solution of gold nanoparticles in colorimetric sensors is used to identify DNA, enzymatic activity, small molecules, metal ions, and proteins. Gold nanoparticles in combination with other molecules can provide a good sensor for detecting target molecules [119]. In optical sensors, silver nanoparticles are more useful because they have a higher absorption coefficient than gold nanoparticles with the same dimensions. Thus, the combination of silver nanoparticles with DNA molecules and other molecules results in the formation of a biodegradable colorimetric biosensor that is used to detect some important analyses. Unlike Au and Ag nanoparticles, Pt and Pd nanoparticles in the visible absorption spectrum do not have surface plasmon resonance (SPR) and therefore are not used in colorimetric sensors [120]. Figure 6 modifies a sensitive, simple colorimetric sensor based on aptameric (Aptameric, oligonucleotides synthesized from RNA or DNA or a combination of the two with other molecules) to detect thrombin (Thrombin, an enzyme needed for blood clotting) using gold nanoparticles. When thrombin is added to a solution of modified gold nanoparticles, thrombin interacts with aptameric on the surface of gold nanoparticles, and after adding a high concentration of sodium chloride salt, the color changes of the gold nanoparticles will be able to detect small amounts of thrombin [41–44].Figure 6
Gold nanoparticle colorimetric process for thrombin detection [30].


## 4.2.3. Noble Metal Nanomaterials for Fluorescence Sensors

Fluorescence sensors are attractive because of their higher sensitivity and better performance than other optical methods. Recent advances in the field of synthesis and characterization of new metal nanomaterials have provided a good basis for the design of biochemical and fluorescence sensors [158]. In general, the design of fluorescence sensors based on metal nanomaterials is done in the following four ways [35, 162, 163]:(A)
The use of fluorescence intensity reduction or “fluorescence quenching” processes of metal nanoparticles by target material molecules: various types of molecular interactions, including excited state reactions, molecular rearrangements, energy transfer, base state complex formation, and collisional or dynamic extinction, can lead to fluorescence extinction. For example, silver nanoparticles are used to detect mercury ions (Hg+2) with low detection limits and high selectivity (Figure 7) [31].(B)
Fluorescence quenching of metal nanoparticles by electron/nonradiative energy transfer mechanism: for example, gold nanoprobes are used to simultaneously identify three analyses of combined adenosine (A), potassium ion (K+), and cocaine, using multiple dyes (Figure 8) [31].(C)
Inner filter effect (IFE) fluorescence based on metal nanoparticles: here, metal nanoparticles act as fluorescence adsorbents for fluorophore emission units (fluorescence generators). It has been shown that gold nanoparticles can be used as a strong adsorbent in IFE fluorescence to detect cyanide (−CN) and hydrogen peroxide (H2O2) (Figure 9) [42].(D)
Metal-enhanced fluorescence (MEF): this means that the emission of fluorophore at a certain distance from metal nanostructures (5–10 nm) can increase. This is an interesting sensor for increasing the detection limit of target molecules (Figure10) [41–44].Figure 7
Fluorescence extinction mechanism for high-sensitivity Hg2 + detection [31].Figure 8
Multicolored gold nanoprobe for simultaneous detection of adenosine, potassium, and cocaine [42].Figure 9
Scheme of IFE-based fluorescence detection process for hydrogen cyanide and hydrogen peroxide [43].
(a)(b)Figure 10
Scheme of metal-enhanced fluorescence (MEF) [44].


## 5. Conclusion

Metal nanostructures with optical, electrical, magnetic, and chemical properties have attracted much research not only scientifically but also in technological applications. To control the morphology of metal nanostructures, special synthesis methods are used, the most important of which is the use of prefabricated patterns or hard molds such as anodic aluminum oxide and soft molds such as cetyltrimethylammonium bromide (CTAB). Important and new applications of metal nanoparticles with different morphology and dimensions are their use in fuel cells, catalysts, and various sensors such as chemical sensors, colorimeters, and fluorescence.



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*Source: 1971891-2022-02-22.xml*