Page 39 - Read Online

Bai et al. Soft Sci 2023;3:40 https://dx.doi.org/10.20517/ss.2023.38 Page 5 of 34

the reducing gases will be more beneficial to the grain boundaries for electron transport, leading to an
increase in conductivity.

Optical induction
The optical property of LMNPs opens the door to optical sensing, the most important of which is the
localized surface plasmon resonance (LSPR) phenomenon. When noble metal NPs are irradiated by light
matching their vibrational frequencies, they will produce a strong absorption of photon energy and result in
LSPR . As a broadband plasmonic material, Ga has a plasmon frequency similar to Al with a bulk plasmon
[85]
energy of up to 14 eV, making Ga an ideal material for achieving LSPR in the ultraviolet (UV) region [1,86] . It
has been shown that the metal oxide (MO) layer on the surface of Ga NPs can increase the mechanical and
chemical stability [87,88] , and the effect of the optical phenomenon of LSPR can be controlled by adjusting the
particle diameter , particle coupling and altering the MO layer . Moreover, surface-enhanced Raman
[89]
[87]
[86]
scattering based on LSPR is also feasible and can be achieved by LSPR-enhanced inelastic light
scattering [90,91] . In addition, some important optical phenomena are worth noting. For example, the dipole
modes of GaNPs can be tuned with the range of spectral regions . The electric coupling substitution
[92]
reaction can visualize the chemistry of GaNPs preparation as an optical tuning phenomenon of the LSPR in
the UV region , and the temperature change can be indicated by the change of extinction cross section at
[93]
the resonance through the solid-core/liquid-shell structure of LMNPs [1,94] . Research on Ga NP
fluorescence , Raman-enhanced Ga plasmon [96-98] , and deposited optically responsive materials is now in
[99]
[95]
full swing, which has led to Ga applications in optical sensors being successfully transformed into a popular
research topic, such as surface plasmon resonance biosensors , surface-enhanced Raman scattering
[100]
sensors [90,101,102] , and photoluminescent biosensors [99,103] .

Biocompatibility
Many sensors are in direct contact with living organisms and should not endanger biological health, so it is
necessary and vital to discuss the biocompatibility of LMs. Typically, macroscopic gallium-based LMs have
been applied to teeth, nerves, and bones as repair materials [104,105] . Because the vapor pressure on the surface
of gallium is negligible at room temperature and almost non-volatile, there is no pathogenic potential from
volatilization and inhalation, as seen with mercury. Also, gallium and its alloys are non-corrosive and non-
permeable for direct contact with living organisms . However, gallium-based LMNPs do not have
[106]
precisely the same properties as macroscopic alloys. Due to their small particle size, LMNPs can be
transported to organs and tissues of living organisms via the reticuloendothelial system with blood [106-108] .
Theoretically, the smaller the volume, the larger the specific surface area and the higher the solubility, while
the reaction of gallium with water produces hydroxy gallium oxide [109,110] , which may lead to the imbalance
of reactive oxygen species [109,111] . Also, the further hydrolytic ionization of hydroxy gallium oxide produces
free gallium ions [109,110] , which may pose a burden risk to organs such as the kidney, liver, and brain of
[112]
organisms . For example, Schedle et al. experimentally found that DNA fragments of murine HL-60 cells
were detected to be broken after incubation in Ga environment at 1 mmol/L for 6 or 72 h, and for L-929
3+
3+
3 H-thymidine, incorporation of fibroblasts was inhibited at Ga concentrations greater than
0.033 mmol/L , suggesting that toxicity is directly linked to dose and that larger doses of LMNPs may lead
[113]
to increased organ burden. However, in most biological experiments, the toxicity of LMNPs was not
manifested, probably due to the low concentration of LMNPs required for the investigation.

At the cellular level, Chechetka et al. tested cell viability after treatment with different concentrations of
LMNPs and other nanomaterials, such as multi-walled CNTs (MWCNTs), single-walled CNTs, and gold
nanorods. LMNPs were found to have a higher survival rate than other nanomaterials under the same
experimental conditions, and the advantage increased significantly with increasing concentrations over the
testing range [Figure 2A i] ; Tang et al. investigated the effect of surface modification on LMNPs toxicity.
[114]