Are Nanoparticles Nanoparticles?

A Look at Gold Particles

A close acquaintance–actually, more like a good friend–asked me to write a few lines about his experiments. He has a background in physics and has developed a method for producing high-purity, atomically dispersed gold particles.

What Are Gold Colloids / Gold Nanoparticles?

Gold colloids are colloidal suspensions of gold nanoparticles (AuNPs), typically ranging in size from a few nanometers to several hundred nanometers.¹ They are therefore approximately one hundred to ten thousand times smaller than human cells and comparable in size to large biomolecules such as enzymes and antibodies. Gold nanoparticles can be synthesized in a wide variety of morphologies, including nanospheres, nanorods, nanoshells, nanostars, and nanocages, and their optical properties can be precisely tuned by controlling particle size, shape, interparticle spacing, and the surrounding medium.

Why Gold as a Carrier Material? ?

Compared with many other nanoparticle systems, gold offers several important advantages, including chemical inertness, low intrinsic toxicity, and straightforward surface functionalization. Silver nanoparticles, for example, readily release potentially toxic silver ions, quantum dots often contain toxic heavy metals, and iron oxide nanoparticles are susceptible to oxidation, which can alter or diminish their functional properties. Gold, in contrast, remains chemically stable under physiological conditions.²

The Central Physical Mechanism: Localized Surface Plasmon Resonance (LSPR)

The remarkable optical behavior of gold colloids is governed by localized surface plasmon resonance (LSPR). When light interacts with a gold nanoparticle, the conduction electrons collectively and coherently oscillate relative to the positively charged metal lattice. If the frequency of the incident light matches the natural frequency of this electron oscillation, resonance occurs, producing a strongly enhanced electromagnetic field at the particle surface.

The resulting plasmon oscillation can decay through two distinct pathways:

• Radiative decay (light scattering): photons of equivalent energy are re-emitted.

• Non-radiative decay (photothermal conversion): absorbed optical energy is converted into heat.

These two processes underpin nearly all biological applications of AuNPs. Nanoparticles with strong scattering properties are particularly useful for imaging techniques such as dark-field microscopy and surface-enhanced Raman spectroscopy (SERS), whereas particles with high absorption efficiencies are employed in photothermal therapy and photoacoustic imaging.

Hypothetical and Proposed Biological Mechanisms

The literature describes several mechanisms through which gold nanoparticles may modulate cellular behavior. Some are supported by substantial experimental evidence, whereas others remain hypothetical.

Spatial Regulators of Receptor Clustering

Gold nanoparticles can function as nanoscale “molecular rulers,” allowing precise control over the spacing between ligands such as epidermal growth factor (EGF) or RGD peptides. Because numerous cell-surface receptors, including the epidermal growth factor receptor (EGFR), require characteristic cluster dimensions (approximately 32–56 nm) for efficient activation, nanoparticle size and geometry may either promote receptor clustering or sterically inhibit it.

Direct Binding to Growth Factors

Unmodified, citrate-stabilized AuNPs have been shown to bind directly to the heparin-binding domain of pro-angiogenic growth factors such as VEGF165. This interaction is proposed to alter the protein’s conformation and consequently suppress its signaling activity, suggesting that structural modification of the growth factor itself may represent the underlying biological mechanism.

Photothermal (Optothermal) Stimulation

When illuminated at their resonance wavelength, plasmonic gold nanoparticles efficiently convert absorbed light into localized heat. This transient heating may reversibly alter the electrical capacitance of the plasma membrane or activate temperature-sensitive ion channels such as TRPV1, thereby inducing membrane depolarization and action potential-like responses. The precise mechanism remains under investigation, and some studies even suggest that membrane proteins may not be strictly required for these effects.

Reactive Oxygen Species (ROS) versus Pure Photothermal Effects

During light-induced cell detachment from gold-coated substrates, two competing mechanistic explanations have been proposed. Some studies attribute the effect primarily to the generation of reactive oxygen species (ROS), implying a photochemical mechanism, whereas others argue that localized heat generation alone is sufficient, representing a purely photothermal process. This illustrates the existence of competing mechanistic hypotheses within the field.

Electrical Conductivity in Nanocomposite Scaffolds

Gold nanoparticles incorporated into tissue-engineering scaffolds have been proposed to improve electrical coupling between cells. For example, it was demonstrated by increasing connexin-43 expression in cardiomyocytes. However, it remains uncertain whether these effects arise from genuine electrical stimulation or from indirect changes in cell-cell interactions facilitated by the presence of the nanoparticles.

Interestingly, Bodelón et al. write:

“The integration of metal nanoparticles in biological systems has triggered a revolution in biology and medicine. Gold nanoparticles are particularly attractive in this respect because they display unique optical properties, low inherent toxicity, large surface area and easy surface functionalization.”

In other terms, one would not expect the same self-restructuring and very problematic mechanisms of LNPs.

Georgeous et al. expand this discussion by adding a mechanistic perspective. They identify and describe several different triggers through which gold nanoparticles could potentially produce therapeutic effects:

Intrinsic Triggers

• pH-Triggered Release: Acid-labile linkages (e.g., amide or hydrazone bonds) are cleaved within the acidic tumor microenvironment or inside lysosomes, resulting in controlled drug release. One example demonstrated approximately 50% doxorubicin release after 96 hours under acidic conditions, compared with negligible release at physiological pH.

• Redox-Triggered Release: Elevated intracellular glutathione (GSH) concentrations, particularly in tumor cells, induce disulfide exchange reactions that destabilize the nanoparticle surface architecture, thereby releasing the associated therapeutic payload.

• Enzyme-Triggered Release: Specific spacer molecules can be cleaved by tumor- or lysosome-associated enzymes (e.g., cathepsins), enabling site-specific drug release within the target tissue.

Extrinsic Triggers

• Light / Near-Infrared (NIR)-Triggered Release: Localized heating generated by laser irradiation at the nanoparticle’s plasmon resonance frequency weakens the interaction between the therapeutic cargo and the particle surface (e.g., during DNA release). This approach provides greater spatial precision and reproducibility than conventional bulk heating.

• Photothermal Therapy (PTT): Unlike triggered drug release, photothermal therapy destroys target cells directly through localized hyperthermia generated by plasmonic nanoparticles, resulting in thermal ablation.

• Photochemical Release: Ultraviolet (UV) irradiation (250–380 nm) cleaves photoresponsive linker groups, thereby activating or releasing previously caged therapeutic molecules.

• Ultrasound-Triggered Release: Ultrasound-induced cavitation near the nanoparticle surface can physically disrupt the interactions between the drug and the particle. A particularly elegant strategy employs gold-loaded microbubbles, which collapse under ultrasound exposure, inducing transient sonoporation of the cell membrane and thereby facilitating cellular uptake of the released nanoparticles.

Thermal Sensitivity of Gold Nanostructures as a Cross-Disciplinary Physical Principle

One property of gold at the nanoscale that appears to be fundamental across multiple fields of research is its pronounced structural sensitivity to temperature.³ In the field of heterogeneous catalysis, Sankar et al. (2020) describe how supported gold species undergo dynamic redistribution during thermal treatment. Atomically dispersed gold, initially trapped within the support material, diffuses outward toward the surface upon heating, while gold already present at the surface simultaneously undergoes diffusion and aggregation. Consequently, the final distribution of gold species, and therefore the catalytic activity of the material, is determined largely by thermally driven processes rather than by the initial synthesis alone.

This thermally induced mobility of gold atoms is not a phenomenon restricted to heterogeneous catalysis, but rather an intrinsic physical property of gold at the nanoscale that is reflected across a wide range of biomedical applications. For example, the synthesis of colloidal gold nanoparticles by the digestive ripening method exhibits a direct and reproducible dependence of particle size on processing temperature, producing particles of approximately 5 nm at 60 °C, while temperatures above 150–180 °C lead to rapid and uncontrolled particle growth (Georgeous et al., 2024). This temperature sensitivity has also been deliberately exploited in nanomedicine. Photothermal drug release is based on the principle that localized heating generated by near infrared irradiation at the plasmon resonance frequency of gold nanoparticles weakens the interaction between the carrier particle and the bound therapeutic agent, thereby enabling controlled drug release (Georgeous et al., 2024; see also Bodelón et al., 2017, for optothermal modulation of neuronal activity and laser induced cell detachment).

Taken together, these findings point to a common physical principle that extends across disciplinary boundaries. Because of their high surface energy and the resulting mobility of surface and interfacial atoms, gold nanostructures exist in a thermodynamically metastable state. Their structural configuration, including atomic distribution, particle size, degree of aggregation, and ligand binding strength, responds sensitively to changes in temperature. In heterogeneous catalysis, this behavior is primarily regarded as a stability challenge because thermally driven sintering and aggregation reduce long term catalytic performance. In contrast, biomedical nanomedicine deliberately exploits the same physical property as a functional mechanism to achieve spatially and temporally controlled therapeutic effects, including controlled drug release and photothermal cell ablation.

Sankar et al.³ note the following regarding gold nanoparticles in the catalytic process:

“In this method, the hydrolysis of urea (CO(NH2)2) permits gradual and homogeneous control of the OH– concentration and avoids local increases in pH and subsequent precipitation of Au(OH)3 away from the support. The DP method is also effective on other types of supports, such as activated carbon, (23) nanodiamond, (24) phosphates, (25,26) hydrotalcite type layered double hydroxides, (27,28) g-C3N4, (29) and hydroxyapatite (HAP). (30) The DP method can even be used to prepare single-atom dispersed Au catalysts. (31) By using UV irradiation instead of a heat treatment, Flytzani-Stephanopoulos and co-workers (32) prepared a Au/TiO2 material with atomically dispersed Au species that was shown to be active for the low-temperature water–gas shift (LT-WGS) reaction. (…)”

“(…) Using cyanide leaching and in situ electron microscopy, Allard et al. (66) demonstrated that a significant amount of atomically dispersed Au can be trapped inside the CP generated support materials, which can subsequently diffuse outward to the surface during subsequent heat treatment (Figure2). This is not too surprising considering the nature of the CP method. He et al. (67) later studied Au on Fe2O3e prepared by the above two CP methods and found that a larger fraction of Au can be trapped into the support when the acidic and basic solutions are mixed quickly (i.e., via Haruta’s original method). The dynamic evolution of Au species during the heat treatment (e.g., via diffusion and aggregation of Au on the surface and via outward diffusion of trapped internal Au species) determines the final population distribution of Au species on the oxide surface as well as the catalytic activity of the material in the CO oxidation reaction.”

Fundamentally Different Material Classes

The fundamental difference is already hidden in the terminology itself: the term “nanoparticle” describes two entirely different physical concepts when comparing atomically dispersed or colloidal gold systems with lipid nanoparticles (LNPs). In the case of gold, the term may refer to atomically purified gold species that exist individually or form colloidal assemblies depending on the surrounding conditions. In contrast, LNPs are not metallic particles but soft, self-assembled lipid structures representing a kinetically stabilized multiparticle system formed by amphiphilic molecules.

AuNPs: AuNPs consist of a solid metallic core composed of gold atoms held together by metallic bonds involving delocalized conduction electrons. They represent an inorganic, crystalline (or atomically dispersed) solid-state system.

LNPs: LNPs are not solid materials but rather self-assembled soft structures composed of amphiphilic molecules, including ionizable lipids, phospholipids, cholesterol, and PEG-lipids. They are held together by weak, noncovalent interactions such as hydrophobic effects, van der Waals forces, and electrostatic interactions. There is no solid “core” in the classical sense; instead, they form micelle- or vesicle-like architectures surrounding encapsulated nucleic acid material such as mRNA.

Different Physical Principles of Action

AuNPs: The central functional principle is localized surface plasmon resonance (LSPR), which describes the collective oscillation of conduction electrons upon interaction with light. This results in either light scattering or photothermal heat generation. This is a purely physical and optical phenomenon arising from the electronic band structure of the metal and can be tuned through particle size and geometry (see Sankar et al. 2020 regarding thermally induced mobility).

LNPs: LNPs do not exhibit a comparable optical resonance mechanism. Their primary mechanism of action is based on membrane fusion and endosomal escape. The ionizable lipids remain largely uncharged at physiological pH but become protonated and positively charged within the acidic environment of the endosome. This charge transition destabilizes the endosomal membrane and enables the release of mRNA into the cytoplasm. This is a chemically driven, pH-dependent mechanism rather than a photophysical process.

Different Stability Mechanisms

AuNPs: Stability and instability are governed by surface energy and atomic mobility. These properties are directly connected to processes such as sintering, Ostwald ripening, and thermally induced diffusion or aggregation of individual gold atoms. The structural state of gold nanostructures is therefore closely linked to their thermodynamic environment.

LNPs: LNP stability is determined by colloidal and physicochemical chemistry, including lipid phase transitions, PEG-mediated steric stabilization against aggregation, and the hydrolytic sensitivity of lipid components. This is one reason why mRNA-LNP formulations often require frozen storage conditions. The primary degradation pathways involve chemical hydrolysis and lipid breakdown rather than atomic diffusion within a solid-state lattice.

Different Loading Mechanisms

AuNPs: Cargo loading occurs through surface chemistry at a solid interface, including covalent Au-S bonds via thiol chemistry, electrostatic adsorption onto the particle surface, or incorporation into porous or hollow structures such as nanocages and nanoshells.

LNPs: Cargo loading occurs through encapsulation within the self-assembled lipid structure during particle formation itself. This is commonly achieved through microfluidic mixing of lipids dissolved in ethanol with mRNA contained in an aqueous buffer solution. The nucleic acid is therefore not attached afterward to a pre-existing surface but becomes integrated as part of the self-assembly process from the beginning.

So much for the basic properties of gold nanoparticles and the underlying scientific considerations. Why am I exploring this topic in the first place? There are some very interesting aspects that seem to offer promising approaches for treatment strategies. However, they also raise the mechanistic questions that will be examined in the following sections.

I won’t recommend selfexperiments, neither I will make any statement “it works”. I can only discuss what I understand, what maybe plausible mechanisms, and formulate a hypothesis:

Hypothesis 1: Local Membrane Temperature Changes as a Recognition Signature of Pathological Cells

One consideration is that cells exhibiting pathological behavior, such as tumor cells with increased aerobic glycolysis (the Warburg effect), altered mitochondrial respiratory chain activity, or elevated production of reactive oxygen species (ROS), may display a measurable, spatially localized thermal signature at the plasma membrane due to their altered metabolism. Since biochemical redox reactions are fundamentally exothermic, increased overall metabolic activity would theoretically be associated with enhanced heat production. Plasmonic gold nanoparticles, whose photothermal properties are known to be sensitive to local thermal environments, could under this hypothesis potentially function as sensor or amplification elements for such localized thermal differences.

This hypothesis conflicts with fundamental thermodynamic considerations. Water, as the dominant intracellular medium, possesses high thermal conductivity, meaning that thermal equilibration at cellular length scales (micrometer range) occurs within microseconds. Under these physical constraints, a stable and sharply localized temperature gradient at the membrane that differs significantly from the rest of the cell would be difficult to maintain. The experimental literature on intracellular thermometry, which in some cases proposes temperature differences of several degrees Celsius within cells, has remained an unresolved controversy within the biophysical community, with experimental evidence and thermodynamic objections both still actively debated.⁴^,⁵^,⁶^,⁷^,⁸

However, regardless of whether a localized thermal gradient at the membrane level exists, the effective accumulation of nonspecifically circulating gold nanoparticles at pathological, particularly neoplastic, cells can be explained through several established mechanisms that are independent of the temperature hypothesis.

The enhanced permeability and retention (EPR) effect describes how tumor vasculature, due to abnormal angiogenesis, exhibits increased vascular permeability combined with impaired lymphatic drainage. As a result, nanoparticles within the size range of AuNPs can passively accumulate within tumor tissue and remain there without requiring any active targeting mechanism (see Georgeous et al., 2024).

In addition, the surface charge of pathological cell membranes is frequently altered. Many tumor cells exhibit an increased negative surface charge compared with healthy cells due to enhanced exposure of anionic phospholipids, such as phosphatidylserine, as well as changes in glycocalyx composition. These alterations may promote electrostatic interactions with the typically slightly negative to neutral surface characteristics of AuNPs.

Furthermore, the increased metabolic activity observed in many tumor cells is often associated with elevated overall endocytic activity. This may increase the probability that nonspecifically associated nanoparticles, or nanoparticles present within the local cellular environment, are internalized even in the absence of active ligand-mediated targeting.

Together, these three mechanisms, passive EPR-mediated accumulation, electrostatic interactions with altered membrane properties, and increased nonspecific endocytic uptake, provide a potential explanation for the enrichment of gold nanoparticles at pathological cells without requiring a localized thermal recognition mechanism.

Therefore, the thermal signature proposed in Hypothesis 1 is neither necessary nor, based on current biophysical understanding, sufficiently supported to explain the effectiveness of nonspecific nanoparticle accumulation. The alternative mechanisms described above provide a substantially more established framework for interpreting such effects.

(Addendum 02.07.2026)

However, the idea of spatial temperature differences between cellular membranes is fascinating: Damaged or pathologically altered cells frequently differ from healthy cells through persistently altered metabolism (e.g., changes in mitochondrial activity, ROS production, ATP turnover, or ion homeostasis). This leads to the hypothesis that such cells may differ not only chemically, but potentially also in their local energy dissipation patterns. If these differences manifest as spatially or temporally persistent thermal microfluctuations, they could theoretically serve as a physical signature of pathological cellular states.

The relevant parameter may therefore not necessarily be the absolute temperature itself, but rather the statistics of local temperature fluctuations, such as their frequency, duration, or spatial correlation. A pathological cellular state might then not be characterized by a permanently elevated temperature, but instead by a distinct pattern of local thermal dynamics.

Established mechanisms of nanoparticle uptake often describe altered membrane properties as a direct determinant of nanoparticle–cell interactions. However, an alternative systems-level perspective is that membrane remodeling itself may represent a downstream consequence of altered cellular signaling states. In this model, pathological influences shift intracellular regulatory networks across functional thresholds, resulting in a new dynamic state characterized by altered membrane organization, receptor clustering, lipid composition, and endocytic activity. Nanoparticle interactions would therefore not arise exclusively from a pre-existing membrane abnormality, but rather from an emergent cellular state in which membrane architecture, signal processing, and physical organization are coupled through feedback mechanisms.

If it were demonstrated that pathological cellular states generate characteristic patterns of local physical fluctuations, this could represent a previously underexplored dimension of nanoparticle interaction. Gold nanostructures would then not necessarily be understood as classical targeting molecules, but potentially as physically responsive probes of altered cellular states.

A possible connection emerges here with established concepts in photobiology and optical modulation of cellular activity. Opsins and other light-sensitive biological systems demonstrate that cells can translate physical stimuli such as light into specific signaling responses. Due to their localized surface plasmon resonance (LSPR), gold nanostructures possess exceptional optical properties, including strong light scattering and photothermal energy conversion. This raises the possibility that they could function as physical amplifiers or transducers capable of transferring local optical or thermal signals into biological contexts.

A particularly intriguing extension of this idea concerns the directionality of such signal transfer. Rather than gold nanostructures merely mimicking light-sensitive biological systems, the more precise question may be whether endogenous photoreceptive proteins, most notably non-visual opsins such as encephalopsin (OPN3), neuropsin (OPN5), or melanopsin (OPN4), could respond to photons scattered or re-emitted by AuNPs through their localized surface plasmon resonance. Contrary to earlier assumptions that confined opsin expression to the retina and specific neural tissues, it is now established that non-visual opsins are expressed across a considerably broader range of tissues and cell types, including skin, adipose tissue, the cardiovascular system, and various non-neuronal cell populations. Since the LSPR scattering spectrum of gold nanostructures is tunable across the visible and near-infrared range through variation of particle size and geometry, and since non-visual opsins possess defined absorption maxima within overlapping spectral regions, a mechanistic coupling between AuNP photon scattering and opsin-mediated intracellular signaling is physically conceivable. In this model, gold nanostructures would not function as autonomous transducers but rather as locally active photon sources that engage pre-existing genetically encoded signal transduction machinery, with the biological specificity of the response determined not by the nanoparticle itself, but by the opsin complement of the target cell.⁹^, ¹⁰^, ¹¹^,

Hypothesis 2: Atomically Dispersed Gold as a Repair-Supporting “Bridge” at Sites of DNA Strand Breaks

A second, equally speculative consideration is the pronounced affinity of gold atoms for sulfur, reflected in the well-established Au-S bond chemistry used in AuNP functionalization (Au-S bond energies approximately 40–50 kcal/mol¹²). Based on this interaction, one could hypothesize that individual, atomically dispersed gold atoms might be attracted to regions of damaged or destabilized DNA and act as a type of structural “bridge” that supports or stabilizes DNA repair processes.

However, DNA itself does not contain sulfur within its sugar-phosphate backbone or within the four standard nucleobases (adenine, thymine, guanine, cytosine). Sulfur-containing groups are primarily found in cellular proteins through cysteine and methionine residues, including many DNA repair enzymes as well as numerous unrelated proteins. Under this assumption, gold would therefore not selectively bind to damaged DNA regions but would instead interact nonspecifically with sulfur-containing proteins throughout the cellular environment.¹³

Furthermore, toxicological and pharmacological studies have shown that the strong thiol affinity of gold compounds often results in inhibition rather than enhancement of enzymatic activity. A well-known example is the inhibition of thioredoxin reductase by the gold-containing rheumatoid arthritis drug auranofin. A specific, repair-promoting interaction with DNA damage sites would therefore likely require deliberate chemical engineering, such as the introduction of artificial thiol groups as used in DNA origami research, and cannot currently be considered a biochemically plausible spontaneous recognition mechanism of free atomic gold.

However, this interpretation may represent a simplified view of the possible biological consequences of gold-sulfur interactions. The strong affinity of gold for sulfur-containing groups does not necessarily imply only direct enzymatic inhibition. In complex biological systems, metal-biomolecule interactions can also influence local molecular geometry, conformational states, and spatial organization of protein-DNA assemblies. Critically, whether a given gold-sulfur interaction is inhibitory or merely modulatory may depend less on the interaction itself than on where, geometrically, the engaged cysteine or methionine residue is positioned within the protein’s three-dimensional architecture. A sulfur residue located within or immediately adjacent to a catalytic center or DNA-binding interface would plausibly disrupt function upon gold coordination, consistent with the auranofin example above. A structurally peripheral residue, by contrast, located away from such functionally critical interfaces, could in principle tolerate gold coordination with little effect on catalysis, or could subtly alter the protein’s conformational geometry and its spatial positioning relative to other repair factors without abolishing its activity.

Therefore, a hypothetical role of atomically dispersed gold would not necessarily require direct binding to DNA bases or a selective recognition mechanism for strand breaks. An alternative possibility is that gold-mediated interactions with sulfur-containing proteins involved in DNA maintenance pathways could, depending on the geometric location of the binding residue relative to catalytic and DNA-binding domains, indirectly affect the structural organization or accessibility of repair complexes, potentially modifying the local repair environment without necessarily abolishing repair function.

Another possibility is a geometric perspective: this reframes the central question from simply whether gold binds sulfur-containing repair proteins to where such interactions occur, how they influence protein structure, and how the location of these interactions relates to functionally critical domains.¹⁴ Multi-protein DNA repair complexes (e.g., base excision repair or nucleotide excision repair machinery) rely on precise spatial coordination between multiple enzymes that assemble transiently at the site of damage. Under this hypothesis, gold coordination at a peripheral, non-catalytic sulfur site could, in principle, subtly alter the conformational presentation or local accessibility of a repair protein within such an assembly, without directly inhibiting its catalytic function, an effect that would be geometry-dependent rather than purely affinity-dependent.

Given this range of possible outcomes, a more defensible formulation of the hypothesis is that gold-sulfur interactions with repair-associated proteins could, depending on the structural location of the engaged residue, either inhibit or subtly modulate the local architecture in which DNA repair occurs, rather than acting uniformly as either a “molecular glue” or a blanket inhibitor. Critically, this reformulation does not rescue the original hypothesis so much as replace it with a more constrained one: any such effect would be indirect, residue-position-dependent, and contingent on structural data that, to date, do not exist for gold-protein interactions within DNA repair complexes specifically. The hypothesis therefore remains untested and should be treated as a structurally plausible but mechanistically unproven possibility, illustrating that the biological consequence of metal-biomolecule interactions cannot be predicted from binding affinity alone, but depends critically on the geometric context in which that binding occurs.

If you wanna know more:

t.me/echtekolloidalebyjoe (German language) Johannes can for sure answer much more questions than I could ever do:

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