Theranostic applications of Raman-active gold nanoparticles

1. Theranostic nanoparticles as an alternative for personalized medicine

Until recently, the standard protocol in medicine was to set a diagnosis and then to implement a therapeutic approach that often involves a drug or a treatment procedure [1]. Medical science has proved that this cannot be longer applied since the most threatening diseases are distinctively heterogeneous and, hence, the capability to provide treatment options that can be efficient in all stages of their progression or for a larger group of patients, is quite limited [2]. Moreover, the more we understand the complexity of diseases like cancer, the more it becomes obvious that the ‘one size fits all’ healthcare concept may actually not fit all [3]. In this regard, the necessity for alternative, more specific and individualized treatment options that combine a therapeutic aspect with diagnosis triggered the development of the so-called “theranostics” or “theragnostics,” linguistic term derived from the Greek words therapia and diagnosis and defined by Funkhouser in 2002 as a combination of therapy modalities and diagnostic imaging in one single platform [4]. A second, simplistic definition implies the application of a specific diagnostic tool to develop a personalized treatment plan [5].

2. Raman-active gold nanoparticles: from particle synthesis to imaging agents

With the emergence of nano-manufacturing technology and due to their capability for probing specific target molecules in complex biological systems, Raman-active or Raman-tagged gold nanoparticles perfectly fit into the category of theranostic compounds. The development of Raman active gold-based nanoparticles for applying theranostic properties has attracted tremendous interest, showing a boom after 2010 [6]. The preference for Raman tags to common fluorescent tags is partially due to their demonstrated photostability and partially due to their capability to generate a multitude of unique spectral signatures while being excited at one or multiple laser wavelengths. As defined by Li et al. [7], Raman reporters may be grouped into three categories: the Surface enhanced Raman Scattering (SERS) probe, the bioorthogonal Raman probe, and the Resonance Raman (RR) probe, the SERS probe being the most attractive due to high enhancement factors (10⁴–10⁸) of the adsorbed reporter molecule. A typical SERS tag consists of a core made of a plasmon resonant metal nanoparticle that generates a local electric field enhancement, a strong Raman scattering molecule (called a Raman reporter) attached on it, and a surface coating (usually a water-soluble polymer) that acts as a particle stabilizer and provides functional sites for further surface modification [8]. The spectral signature used to identify a biomolecular interaction is generated by the Raman reporter molecule when the particle-molecule complex is excited with laser light. The ability of a SERS tag to be practically applicable in ultrasensitive detection and imaging is fundamentally linked to its inner capacity for Raman signal generation, without the requirement of an external signal transduction via plasmonic coupling with a supplementary underlying metal substrate. Moreover, biomolecules can be incorporated in the complex for targeting specific receptors, usually the design being based on immune-labeling.

For an optimal SERS signal, the Raman reporter has to be either adsorbed or located within a few nanometers range from the nanoparticle surface because both the chemical and electromagnetic enhancement mechanisms are strongly distance-dependent [9]. Further on, for an effective intracellular or in-tissue detection, the SERS tag has to be bright but morphologically designed in a way that would not interfere with the natural biological processes. Pioneering research on such SERS-active bioanalytical tools was performed in 2003, with the independent works of Mulvaney et al. [10] and Doering and Nie [11]. Recent progress on SERS tags showed that improved functionalities can be obtained by incorporating more than one Raman reporter molecule inside the same compound or by using stimuli responsive coatings for their encapsulation [12]. Moreover, multiplexed cancer imaging and detection, similarly to the fluorescent dyes can be achieved by employing SERS tags in innovative diagnostic protocols [13]. The high information content of the Raman spectrum can provide scientists with essential information related to the intracellular pharmacokinetics or cellular processes and functions.

3. Finding the best particle option

Based on the application, the rational design and synthesis of metal nanoparticles can be finely guided since their optical properties are determined by a set of parameters that include composition, particle size and shape, overall architecture, and local environment [14]. Gold nanoparticles represent excellent surface enhancers for Raman scattering of the adsorbed molecules and this property, combined with the general biocompatibility of Au, are enabling their use in the areas of human health and medicine. However, a set of parameters have to be taken into account when seeking for the best particle option. A first logical step is to select the composition and regarding this, Au is one of the most studied materials and the evident choice for stability and nontoxicity as compared to other metal enhancers [15]. High-quality nanoparticles can be synthesized from Au but most importantly, at the nanoscale, this material has peak plasmon frequencies in the visible region of the spectrum where the resonant condition for polarizability is met and the electromagnetic field is enhanced [16]. The importance of particle size and shape comes into account, especially when the aim is to extend the localized surface plasmon resonance (LSPR) maximum into the near-infrared biologically transparency window [17]. Tremendous attention was given to the development of various nanoparticle architectures and for decoupling the intertwined roles of core material, seed morphology, capping agent, and growth kinetics as regarding the control of the particle shape [18]. Finally, the sensitivity to changes in the local environment would guide the choice of the capping agent for providing chemical stability and therapeutic functionality to the nanoparticle-system.

4. Instead of conclusions: how can Raman-active gold nanoparticles be implemented into the clinic?

Questions as: “Is there an effective and economical way to provide personalized treatment? What are the costs? What about timing-diagnostic/effectiveness of the treatment?” come into spotlight when the forehead vision is to implement Raman-active gold nanoparticles into the treatment protocols used in the clinic. Current trends imply Raman spectroscopy in clinical settings, especially by the use of portable Raman spectrometers, and mostly for diagnosis purposes [19]. However, as regarding the Raman-active products, their real implementation would firstly require the development of new tags that exhibit signal in the silent-cell spectral region (1800–2800 cm⁻¹) which can help overcome the inherently poor signal-to-noise ratio and the presence of overlapping Raman bands in measurements of biological samples [20]. Moreover, the employment of the two clear windows (NIR-I: 650–950 nm and NIR-II: 1000–1700 nm) is preferred both for in vivo molecular imaging and for therapeutic purposes (e.g., photothermal therapy, treatment efficiency screening) since operating in these wavelength windows would yield improved detection sensitivity, enhanced spatial resolution, higher tissue penetration depth and minimized background autofluorescence [21]. Next, safety issue must be carefully considered as the enhanced reactivity of nanoparticles caused by their high surface-area-to-volume ratio lends far greater complexity and potential toxicity [22]. Therefore, all the guidelines’ requirements and regulations imposed for the validation of a therapeutic product must be accomplished in order to qualify for clinical use.

Funding Statement

This work was supported by the MCID through the ‘Nucleu’ Program within the National Plan for Research, Development, and Innovation 2022–2027, project PN 23 24 01 02.

Author contributions

This work was authored solely by Sanda Boca.

Disclosure statement

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.