Why did we choose not to sell them "bare"? 

If you have ever worked with gold nanostars, you may have observed a frustrating phenomenon: nanostars that initially exhibit a strong LSPR band in the 900–1100 nm region (ideal for imaging or photothermal applications) gradually develop a blue-shifted spectrum, eventually approaching that of spherical gold nanoparticles with a peak around 520–580 nm.

If you could follow this process by TEM, you would see the underlying structural changes: progressive erosion of the branches, which become shorter and more rounded over time, turning well-defined stars into raspberry-like particles and, ultimately, into imperfect nanospheres (as illustrated in the figure).

Instability Mechanisms of Bare Nanostars

Biological systems are crowded, salty, and full of sticky macromolecules. A bare gold nanostar that dropped into this environment participates in many unnecessary interactions that good surface chemistry can prevent.

*         Nonspecific protein adsorption (uncontrolled protein corona): Bare or weakly coated gold strongly attracts serum proteins like albumin and many others, leading to thick, complex coronas that are hard to predict and reproduce. This can mask targeting ligands, alter cellular uptake, and change immune recognition.

*         Salt‑induced aggregation: Physiological ionic strength screens charges and promotes van der Waals attraction between nanostars, especially along their high‑curvature tips, causing clustering and sedimentation.

*         Unwanted cell membrane interactions: Bare gold can interact strongly with lipid membranes, potentially causing membrane disruption, off‑target uptake, or toxicity that is unrelated to the intended mechanism of action.

*         Bridging by biomolecules: Multivalent proteins or peptides can crosslink uncoated nanostars, forming large aggregates rather than well‑behaved single particles.

In other words, bare nanostars may survive in a clean cuvette, but they rarely survive the journey into realistic life‑science environments, being unstable in storage, unpredictable in biological media, and prone to the wrong interactions with cells and proteins. In contrast, dense PEG or zwitterionic coatings are known to strongly reduce nonspecific protein adsorption and corona formation, giving a more “stealthy” and predictable nanoparticle in blood or cell culture media.

Evidence from Scientific Literature

Numerous studies demonstrate that surface coatings are not cosmetic; they are essential for stability and reliable bio‑performance.

*         PEG-functionalized gold nanoparticles resist aggregation during aging, thermal stress, and autoclaving, unlike unprotected particles.

*         Under-PEGylated gold nanostars aggregate and remain immobile in gels, while fully coated ones exhibit sharp, mobile bands indicative of discrete particles.

*         Functionalization with PEG, peptides, or biomolecules preserves optical stability and colloidal integrity for biosensing and bioimaging.

*         Bare gold colloids aggregate easily, with protein coronas sometimes providing unintended shielding.

Together, these examples tell a consistent story: whenever researchers want nanostars or related gold nano‑objects to behave reproducibly in complex media, they wrap them in a carefully designed coating layer.

How smart coatings change the story

Engineered coatings transform fragile bare nanostars into reliable platforms for life‑science applications.

*         Stealth and colloidal stability: E.g. PEG or zwitterionic ligands create a hydrated, steric barrier that suppresses nonspecific protein binding and keeps nanostars dispersed over time, even in serum and at 37 °C.

*         Tunable bio‑interface: By coupling peptides (e.g., cell‑penetrating sequences like TAT) or targeting ligands onto an underlying stabilizing layer, researchers have increased intracellular delivery and photothermal efficiency while maintaining acceptable cell viability.

*         Preserved optical performance: Coated gold nanostructures (nanostars, nanorods, nanospheres) maintain narrow, well‑defined LSPR peaks, which is crucial for applications like SERS, photothermal therapy, or imaging where spectral position and intensity matter.

*         Reduced toxicity from synthesis surfactants: In systems analogous to nanostars, coatings and ligand are used to replace cytotoxic surfactants (such as CTAB), enabling in‑body use.

A good illustration is the work where increasing PEG coverage on gold nanostars transformed them from immobile, aggregated species to discrete, fast‑moving bands in electrophoresis, indicating a complete PEG monolayer and robust colloidal stability suitable for bioconjugation.

Our Approach: Coated Nanostars Only

Given this evidence, offering bare gold nanostars for life‑science use would mean introducing a known source of instability and variability.

Instead, we focus on nanostars with rationally designed surface coatings that deliver:

  • Resistance to agglomeration and morphological degradation in buffers or biological media.
  • Reduced nonspecific protein adsorption and unintended cellular interactions.
  • Consistent LSPR performance for NIR-targeted applications.
  • A reliable base for further functionalization with antibodies, peptides, or oligonucleotides.

At NanoBrand, we offer gold nanostars coated with chitosan (positive surface charge) or PVP (negative surface charge), and we provide customized coatings tailored to your specific applications. To learn more, please visit our Gold Nanostars products.

References:

1. Tukova, A., Nie, Y., Tavakkoli Yaraki, M., Tran, N. T., Wang, J., Rodger, A., Gu, Y., & Wang, Y. (2023). Probing Protein Corona Formation around Gold Nanoparticles: Effects of Surface Coating. Aggregate, 4, e323. https://doi.org/10.1002/agt2.323 

2. Ashkarran, A. A., Tadjiki, S., Lin, Z., Hilsen, K., Ghazali, N., Krikor, S., Sharifi, S., Asgari, M., Hotchkin, M., Dorfman, A., Ho, K. S., & Mahmoudi, M. (2024). Protein Corona Composition of Gold Nanocatalysts. ACS pharmacology & translational science, 7(4), 1169–1177. https://doi.org/10.1021/acsptsci.4c00028  

3. Nguyenova, H. Y., Hubalek Kalbacova, M., Dendisova, M., Sikorova, M., Jarolimkova, J., Kolska, Z., Ulrychova, L., Weber, J., & Reznickova, A. (2024). Stability and biological response of PEGylated gold nanoparticles. Heliyon, 10(9), e30601. https://doi.org/10.1016/j.heliyon.2024.e30601

4. Yokoyama, K., Barbour, E., Hirschkind, R., Martinez Hernandez, B., Hausrath, K., & Lam, T. (2024). Protein Corona Formation and Aggregation of Amyloid β 1-40-Coated Gold Nanocolloids. Langmuir : the ACS journal of surfaces and colloids, 40(3), 1728–1746. https://doi.org/10.1021/acs.langmuir.3c02923

5. Dallari, C., Capitini, C., Calamai, M., Trabocchi, A., Pavone, F. S., & Credi, C. (2021). Gold Nanostars Bioconjugation for Selective Targeting and SERS Detection of Biofluids. Nanomaterials (Basel, Switzerland), 11(3), 665. https://doi.org/10.3390/nano11030665

6. Shi, X., Perry, H. L., & Wilton-Ely, J. D. E. T. (2021). Strategies for the functionalisation of gold nanorods to reduce toxicity and aid clinical translation. Nanotheranostics, 5(2), 155–165. https://doi.org/10.7150/ntno.56432

7. Yuan, H., Fales, A. M., & Vo-Dinh, T. (2012). TAT peptide-functionalized gold nanostars: enhanced intracellular delivery and efficient NIR photothermal therapy using ultralow irradiance. Journal of the American Chemical Society, 134(28), 11358–11361. https://doi.org/10.1021/ja304180y

8. Foti, A., Clépoint, B., Fraix, A., D'Urso, L., De Bonis, A., & Satriano, C. (2024). A simple approach for CTAB-free and biofunctionalized gold nanorods to construct photothermal active nanomedicine for potential in vivo applications in cancer cells and scar treatment. Frontiers in Materials, 11, 1381176. https://doi.org/10.3389/fmats.2024.1381176

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