Gold nanoparticles (GNPs) and GNP-based multifunctional nanocomposites are the subject of intensive studies and biomedical applications. This minireview summarizes our recent efforts in analytical and theranostic applications of engineered GNPs and nanocomposites by using plasmonic properties of GNPs and various optical techniques. Specifically, we consider analytical biosensing; visualization and bioimaging of bacterial, mammalian, and plant cells; photodynamic treatment of pathogenic bacteria; and photothermal therapy of xenografted tumors. In addition to recently published reports, we discuss new data on dot immunoassay diagnostics of mycobacteria, multiplexed immunoelectron microscopy analysis of Azospirillum brasilense, materno-embryonic transfer of GNPs in pregnant rats, and combined photodynamic and photothermal treatment of rat xenografted tumors with gold nanorods covered by a mesoporous silica shell doped with hematoporphyrin.

Au, Ag, Se, and Si nanoparticles were synthesized from aqueous solutions of HAuCl₄, AgNO₃, Na₂SeO₃, and Na₂SiO₃with extra- and intracellular extracts from the xylotrophic basidiomycetesPleurotus ostreatus,Lentinus edodes,Ganoderma lucidum, andGrifola frondosa. The shape, size, and aggregation properties of the nanoparticles depended both on the fungal species and on the extract type. The bioreduction of the metal-containing compounds and the formation rate of Au and Ag nanoparticles depended directly on the phenol oxidase activity of the fungal extracts used. The biofabrication of Se and Si nanoparticles did not depend on phenol oxidase activity. When we used mycelial extracts from different fungal morphological structures, we succeeded in obtaining nanoparticles of differing shapes and sizes. The cytotoxicity of the noble metal nanoparticles, which are widely used in biomedicine, was evaluated on the HeLa and Vero cell lines. The cytotoxicity of the Au nanoparticles was negligible in a broad concentration range (1–100 µg/mL), whereas the Ag nanoparticles were nontoxic only when used between 1 and 10 µg/mL.

Recent reports have shown significant improvement of the polymerase chain reaction (PCR) efficiency and specificity with the addition of gold nanoparticles (AuNPs). However, the underlying mechanisms are still unclear and debatable. Here, we report effects of AuNP concentration, shape, material, and surface functionalization on PCR outcome under typical amplification conditions. All nanoparticles were tested with two PCR diagnostic models: (1) the nitrogen fixation (NifD) gene from Azospirillum brasilense Sp7 bacteria and (2) the polymorphic ompA gene encoding major outer membrane protein of Chlamydia trachomatis. The optimal concentration of 16 nm citrate-stabilized negatively charged spherical AuNPs (zeta potential −29.9 mV) was determined to be 0.4 nM, in agreement with previous data for quite different PCR systems. Functionalization with poly(vinylpyrrolidone) (PVP) and thiolated PEG (mPEG-SH) essentially did not change the zeta-potential (−30 mV and −21 mV, respectively), but completely suppressed the PCR enhancement. Positively charged AuNPs with poly(diallyldimethylammonium) (PDDA) and cetyltrimethylammonium bromide (CTAB) capping polymers exhibited no specific enhancing effect on PCR or even inhibited the reaction at 5 pM of AuNP–PDDA. Also, no specific PCR enhancement was obtained with CTAB-stabilized gold nanorods. To elucidate the role of the particle material, we performed PCR with gold and silica particles of comparable size and equal negative charges. In contrast to the great PCR enhancement with 16 nm AuNPs, 20 nm silica nanoparticles did not affect the PCR efficiency or inhibit the reaction at concentrations higher than 100 nM. These findings suggest that the nature of the PCR system components, surface stabilizing agents and the thermal conductivity of the nanoparticle material can play important roles in the nanoparticle-mediated PCR.

We introduce a new genosensing approach employing CTAB (cetyltrimethylammonium bromide)-coated positively charged colloidal gold nanoparticles (GNPs) to detect target DNA sequences by using absorption spectroscopy and dynamic light scattering. The approach is compared with a previously reported method employing unmodified CTAB-coated gold nanorods (GNRs). Both approaches are based on the observation that whereas the addition of probe and target ssDNA to CTAB-coated particles results in particle aggregation, no aggregation is observed after addition of probe and nontarget DNA sequences. Our goal was to compare the feasibility and sensitivity of both methods. A 21-mer ssDNA from the human immunodeficiency virus type 1 HIV-1 U5 long terminal repeat (LTR) sequence and a 23-mer ssDNA from the Bacillus anthracis cryptic protein and protective antigen precursor (pagA) genes were used as ssDNA models. In the case of GNRs, unexpectedly, the colorimetric test failed with perfect cigar-like particles but could be performed with dumbbell and dog-bone rods. By contrast, our approach with cationic CTAB-coated GNPs is easy to implement and possesses excellent feasibility with retention of comparable sensitivity--a 0.1 nM concentration of target cDNA can be detected with the naked eye and 10 pM by dynamic light scattering (DLS) measurements. The specificity of our method is illustrated by successful DLS detection of one-three base mismatches in cDNA sequences for both DNA models. These results suggest that the cationic GNPs and DLS can be used for genosensing under optimal DNA hybridization conditions without any chemical modifications of the particle surface with ssDNA molecules and signal amplification. Finally, we discuss a more than two-three-order difference in the reported estimations of the detection sensitivity of colorimetric methods (0.1 to 10-100 pM) to show that the existing aggregation models are inconsistent with the detection limits of about 0.1-1 pM DNA and that other explanations should be developed.