Silver Nanoparticle Properties

1.Background

Silver nanoparticles are metallic silver particles with a particle size of less than 100nm, which have significant characteristics: surface effects result in a large specific surface area and high surface activity; The quantum size effect brings unique optical and electrical properties; The small size effect changes its physical properties such as melting point and hardness; The macroscopic quantum tunneling effect is beneficial for the application of electronic devices. It has a wide range of applications: in medicine, it is used as a broad-spectrum antibacterial agent, wound dressing, etc., and can also promote tissue repair; In the field of microelectronics, it is used as a conductive material for electrodes, integrated circuits, etc; Optoelectronics can produce sensors and SERS substrates; Catalytic field can improve reaction efficiency.

Figure 1 presents the plasmon resonance spectral characteristics of Cytodiagnostics silver nanoparticles with different sizes as measured by ultraviolet-visible spectroscopy: as the particle size increases from 10 nm to 100 nm, the absorbance peak (¦Ëmax) red-shifts from 400 nm to 500 nm, and the peak width gradually broadens. It is worth noting that for larger particles (especially those above 80 nm), in addition to the main dipole resonance peak, an obvious secondary peak appears at a lower wavelength, which is caused by the quadrupole resonance effect.

Figure 1. Silver nanoparticle size-dependant surface plasmon resonance. Note the red-shift of the absorption maximum as the gold nanoparticle size increases.

2.Factors affecting spectral characteristics

Note: Surface Plasmon Resonance (SPR): Silver nanoparticles exhibit significant surface plasmon resonance properties, which are caused by the collective oscillation of their surface-free electrons under incident light. When the frequency of the incident light matches the oscillation frequency of surface plasmons, intense absorption and scattering occur, resulting in an absorption peak at a specific wavelength. Silver nanoparticles of different shapes and sizes have different SPR absorption peak positions.  

1. Intrinsic properties of silver particles  

a. Size  

The size of silver nanoparticles has a significant impact on their spectral properties. On one hand, as the particle size increases, the surface plasmon resonance (SPR) peak undergoes a red-shift. According to Mie scattering theory, larger particles are more prone to multiple scattering, causing the SPR peak to shift toward longer wavelengths. On the other hand, the particle size affects the efficiency of scattering and absorption: small-sized particles are dominated by absorption, while the scattering effect of large-sized particles is enhanced.  

b. Shape  

Silver nanoparticles of different shapes have distinct surface plasmon resonance modes, which in turn affect their spectral properties.  

c. Concentration  

The concentration and aggregation state of silver nanoparticles are of great significance for their spectral characteristics and stability monitoring:

When silver nanoparticles are at low concentrations, the interaction between particles is weak, and the spectrum can remain relatively stable; however, as the concentration increases, the coupling effect between particles strengthens, leading to broadening or shifting of the surface plasmon resonance (SPR) peak. Meanwhile, the aggregation state of silver nanoparticles can also significantly change their optical properties. This characteristic can be used to monitor the stability of nanoparticles ¡ª whether it is the natural change over time or the state change after adding salt-containing buffer solution (which can induce particle aggregation when its concentration is sufficiently high), it can be reflected through the spectrum. For example, the absorption spectrum in Figure 2 shows that when aggregation occurs, the main peak weakens and the absorption in the red light region enhances.

Figure 2. Visual appearance and UV-VIS spectra of monodisperse (A) and sodium chloride (NaCl) induced agglomeration (B) of 10nm silver nanoparticles.

3. Other properties

A. Electrical Properties  

Quantum size effect: When the size of silver nanoparticles is reduced to a certain extent, the energy levels of electrons become discrete, resulting in the quantum size effect. This makes the electrical properties of silver nanoparticles significantly different from those of bulk silver.  

B. Thermal Properties  

Change in thermal conductivity: Compared with bulk silver, the thermal conductivity of silver nanoparticles undergoes a significant change.  

This is mainly due to the alteration of phonon scattering mechanisms at the nanoscale. The presence of surfaces and interfaces hinders the propagation of phonons within silver nanoparticles, leading to a reduction in thermal conductivity.  

C. Catalytic Properties  

High catalytic activity: Silver nanoparticles exhibit high catalytic activity, which may stem from the injection of hot electrons generated by surface plasmon resonance or the adsorption and activation of reactants by surface active sites. This is benefited from their large specific surface area and the high reactivity of surface atoms.  

D. Antibacterial Properties  

Broad-spectrum antibacterial activity: Silver nanoparticles have antibacterial activity against a variety of pathogens, including Gram-negative bacteria (such as Escherichia coli) and Gram-positive bacteria (such as Staphylococcus aureus). Meanwhile, some specially prepared and modified silver nanoparticles possess good antibacterial persistence.

Influence of Size Regulation on SPR Performance and Its Implementation Methods  

Influence of size on SPR performance:

The size of nanoparticles has a significant impact on the SPR phenomenon. Simulation studies on Ag nanoparticles of different sizes using the Finite-Difference Time-Domain (FDTD) method show that as the particle size increases, the intensity of absorption and extinction peaks increases, accompanied by a blue shift, while the scattering factor undergoes a red-shift, with the peak intensity still increasing significantly. This means that by controlling the size of nanoparticles, their response to specific signals in biosensing can be adjusted in a targeted manner. For example, in biosensing applications requiring enhanced absorption signals, the size of nanoparticles can be appropriately increased.  

Methods to achieve size regulation:  

Laser energy regulation: When synthesizing silver nanoparticles by pulsed laser ablation of silver immersed in distilled water, the incident laser energy has a significant impact on their size.  

Optimization of chemical reduction conditions: In the synthesis of silver nanoparticles using the chemical reduction method, various reaction conditions can affect their size. In addition, the size of nanoparticles can also be regulated by changing conditions such as reducing agent concentration and reaction temperature.  

4.Influence of Morphology Regulation on SPR Performance and Its Implementation Methods  

Influence of morphology on SPR performance:

Silver nanoparticles have diverse morphologies, and different morphologies lead to differences in their SPR performance. For example, anisotropic silver nanorods and nanowires, as well as dendritic silver nanoparticles, silver nanoplates (disks), silver nanocubes, and triangular prisms, etc., their unique structures endow them with different optical properties. In biosensing, nanoparticles of different morphologies have different adsorption capacities and modes for biomolecules, thereby affecting the changes in SPR signals. For instance, the larger surface area of silver nanoplates enables them to adsorb biomolecules more effectively, enhancing the SPR signal and improving sensing sensitivity.  

Methods to achieve morphology regulation:  

Surfactant induction: Using the liquid-phase reduction method, the growth process of silver nanocrystals can be regulated by adding various surfactants, thereby preparing silver nanoparticles with different morphologies.  

Soft template technology: Soft template technology can be used to synthesize silver nanoparticles of various morphologies. This technology utilizes soft templates with specific structures to guide the growth of silver ions in specific directions, thereby forming anisotropic nanostructures.

[1] Hordovska N, Koidan A, Lozovski V, et al. Changes in Optical Properties of Bacteria with Silver Nanoparticles Caused by Their Interaction[C]//2022 IEEE 41st International Conference on Electronics and Nanotechnology (ELNANO). IEEE, 2022: 435-439.

[2] Xu J, Wang K, Li Y, et al. Regulating silver nanowire size enables efficient photoelectric conversion[J]. Science China Chemistry, 2020, 63: 1046-1052.

[3] Ibrayev N, Afanasyev D, Ishchenko A, et al. Influence of silver nanoparticles on the spectral-luminescent and lasing properties of merocyanine dyes solutions[J]. Laser Physics Letters, 2021, 18(8): 085001.

[4] Samusev I G, Tikhomirova N S, Slezhkin V A, et al. Silver nanoparticles plasmonic effect on eosin and rhodamine 6G luminescence in various media[C]//Nanophotonics and Micro/Nano Optics III. SPIE, 2016, 10027: 138-143.

[5]Weerasekera R, Samarakoon D, Dayalal D, et al. Mie-theory-based investigation of the effect of size distribution on the surface plasmon absorption of silver nanoparticles[C]//2022 Moratuwa Engineering Research Conference (MERCon). IEEE, 2022: 1-6.