This study introduces a dual-particle emulsion design that separates sensing and stabilization roles, enabling robust SERS detection in complex biological and environmental samples.
(Nanowerk Spotlight) Detecting trace molecules in complex samples such as blood or wastewater remains a critical challenge in analytical chemistry—one that surface-enhanced Raman spectroscopy (SERS) is uniquely suited to address through its ability to amplify molecular fingerprints near plasmonic surfaces.
By magnifying Raman scattering signals through plasmonic nanostructures—typically noble metal particles like gold or silver—SERS enables identification and quantification of substances down to single-molecule levels. This capability has made it appealing in biomedical diagnostics, forensic science, and environmental monitoring.
However, practical use of SERS in complex biological or environmental samples has been limited by two persistent problems: chemical instability of the enhancing materials and the need to balance surface accessibility with structural integrity.
One format for SERS enhancement is the Pickering emulsion: a system where droplets of one liquid are stabilized in another immiscible liquid using solid particles. These emulsions create extended interfaces that allow analyte molecules to interact with plasmonic particles, making them potential platforms for interfacial detection.
But until now, the same particles responsible for plasmonic activity were also required to stabilize the emulsions. This created a fundamental tradeoff. Modifying nanoparticle surfaces to stabilize emulsions often blocked access to analytes, while analyte binding could destabilize the emulsions. These issues were particularly acute in biological samples, which contain competing molecules that interfere with nanoparticle surface chemistry.
To address this, a research team from East China University of Science and Technology and Queen’s University Belfast developed a dual-particle Pickering emulsion system. Described in Advanced Science (“Untying Surface Chemistry and Emulsion Stability to Construct Multifunctional Pickering Emulsion SERS Sensors for Pretreatment‐Free Quantitative Analysis in Bio‐Media”), their system separates the roles of stabilization and sensing, allowing independent tuning of surface functionality and emulsion stability. Multiwalled carbon nanotubes (MWCNTs) serve as stabilizers, while gold nanoparticles coated with a porous Prussian blue (Au@PB) shell provide the plasmonic functionality. This decoupling enables robust, multifunctional SERS sensors capable of quantitative, biphasic, and multiplexed analysis directly in complex samples.
Au@PB nanoparticles and their assembly into Pickering emulsions. (A) UV-vis spectra of Au and Au@PB colloid. (B) HRTEM showing the PB shell layer and typical Au@PB nanoparticles and agglomerates. (C) HAADF-EDX analysis of a typical Au@PB nanoparticle. The scale bars in (B-C) correspond to 10 and 20 nm, respectively. (D) Schematic illustrations of the synthetic procedure of MWCNT-Au@PB Pickering emulsions. (E) Schematic illustrations of the MWCNT stabilizer increasing the maximum capillary pressure that could be withstood between emulsion droplets. (F) Schematic illustration of the promoter acting as a charge-screening agent in interfacial self-assembly. (G) The molecular structure of the amphiphilic TBA+ NO3− promoter. (H) Three-phase contact angle measurements of Au@PB and MWCNT-Au@PB in water-cyclohexane. (Image: Reprinted from DOI:10.1002/advs.202505714, CC BY) (click on image to enlarge)
The Au@PB particles were synthesized through a controlled chemical deposition process that forms a thin, porous Prussian blue layer around gold cores. This layer admits small analytes while excluding larger interfering molecules, such as proteins. Simultaneously, the gold core enhances Raman signals. MWCNTs, dispersed in oil, anchor at the water-oil interface to stabilize emulsion droplets. Aided by tetrabutylammonium nitrate (TBA+NO3⁻), which reduces repulsion between negatively charged nanoparticles, the system forms a stable emulsion where Au@PB particles concentrate at droplet interfaces and create SERS hotspots.
Stability tests showed that emulsions remained intact for over a month when MWCNT concentrations were above 0.05 mg/mL. Droplet size and stability were tunable by adjusting MWCNT amounts and lengths. At lower concentrations, the emulsions became unstable, coalescing into a continuous phase. These structural features were confirmed using microscopy and provided a foundation for controlled SERS sensing.
The system’s performance was evaluated using crystal violet, a standard analyte. Strong SERS signals were observed, and the Prussian blue shell served as an internal reference to calibrate intensity fluctuations. This reduced signal variation across droplets and batches, with relative standard deviations falling to 12.6%. The system’s robustness and reproducibility made it suitable for practical quantitative analysis.
The researchers demonstrated detection of a broad range of analytes, including pesticides, neurotransmitters, pollutants, and pharmaceuticals. These included both water- and oil-soluble compounds, highlighting the biphasic capabilities of the emulsions. For example, dopamine and 4-mercaptobenzoic acid were introduced in different phases and detected simultaneously within a single sample.
A key test involved detection of adenine in artificial serum. Traditional SERS emulsions fail in such samples because proteins displace surface modifiers and destabilize the emulsion. Here, the MWCNTs provided stability while the PB shell excluded large proteins, enabling selective detection of small analytes. Adenine was detectable down to 10⁻⁸ M, and reproducibility was improved further by using the PB shell as a calibration standard. These features allowed accurate and consistent quantification, overcoming a major hurdle in real-world SERS applications.
The emulsions also enabled multiplex detection. The researchers simultaneously detected dopamine and melamine in serum, with clear, separate spectral peaks. This capability extended to environmental samples containing both hydrophilic and hydrophobic analytes, using different oil phases to optimize solubility and signal clarity.
This study introduces a modular design for constructing biphasic SERS sensors with independently tunable properties. By assigning stabilization and sensing to separate nanoparticle types, the researchers created a flexible interface architecture that supports analyte access, signal amplification, and long-term structural resilience. These emulsions are compatible with a variety of oils and analytes, suggesting adaptability for different sensing contexts.
While the platform is versatile, its current implementation relies on lab-based synthesis and detection techniques. Future work could explore scale-up methods, in vivo compatibility, and integration with portable Raman devices. The underlying strategy—decoupling stability from functionality—opens possibilities for tailored nanomaterial systems in diagnostic and environmental monitoring settings.
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