Plasmonic Plastic Nanocomposites for Gas Sensing

Nanoplasmonic systems based on localized surface plasmon resonance (LSPR) have revolutionized various technologies, ranging from plasmonic sensors and photocatalysts to photovoltaic devices, plasmonic lasers, and optical metamaterials. Particularly in the field of sensors, nanoplasmonic systems have played a pivotal role, enabling the detection of biological analytes, monitoring water and food contamination, and detecting gaseous species like H2, NO₂, CO, and CO₂.  Additive manufacturing, also known as 3D printing, has emerged as a transformative method for creating complex 3D objects. Traditionally, it has been applied to neat materials such as metals and polymers. However, recent advancements have focused on nanocomposites, which consist of nanoparticles dispersed within a host matrix. This approach allows for precise engineering of material properties at the nanoscale. The intersection of additive manufacturing and nanoplasmonics presents exciting opportunities for scalable plasmonic material synthesis and cost-effective device integration. This approach has the potential to usher in a paradigm shift in the field of plasmonics, as it enables the creation of “plasmonic plastics.” These plasmonic plastic nanocomposites consist of colloidal metal nanoparticles with sensor functionality dispersed in a polymer matrix. They not only facilitate the additive manufacturing of plasmonic devices but also act as molecular filters, enhancing sensor selectivity and enabling operation in chemically challenging environments.

A new study published in Accounts of Chemical Research Journal led by Iwan Darmadi, Ida Östergren, Sarah Lerch, Anja Lund, Kasper Moth-Poulsen, Christian Müller, and Christoph Langhammer, developed a novel approach to plasmonic sensing through the utilization of additive manufacturing and the development of “plasmonic plastics.” The authors discussed the key discoveries, rational design, and optimization of plasmonic plastic nanocomposites and their constituents, including colloidal plasmonic nanoparticles and the polymer matrix. Emphasis is placed on materials design grounded in a fundamental understanding of the limiting factors for the targeted applications. Furthermore, the authors demonstrate how synthesis and processing of these nanocomposites can be scaled up to kilogram quantities using flow synthesis of colloidal nanoparticles and large-scale polymer compounding infrastructure.

The conceptual framework for plasmonic plastic nanocomposites in gas sensing involves three crucial components: plasmonic metal nanoparticles, surfactant molecules on nanoparticle surfaces, and the polymer matrix.

Different metals can be chosen to prepare nanoparticles based on the desired sensing functionalities. These metals can be utilized for both direct and indirect plasmonic sensing schemes. In direct sensing, the nanoparticles serve as both the sensing material and the plasmonically active transducer. Examples include H₂-absorbing Pd and oxidizing Al and Cu nanoparticles. In indirect sensing, the sensing material and the plasmonic transducer are separate, with one being a plasmonic nanoparticle probe and the other a closely adjacent sensing material. The choice of nanoparticle size, shape, and composition can be tailored to control the spectral position of the plasmonic resonance and surface chemistry. The alignment of anisotropic nanoparticles, like nanorods, significantly impacts the optical properties of the nanocomposite.

Surfactant molecules on the surface of colloidal nanoparticles are essential for preventing aggregation in solution and promoting shape-specific particle growth. The choice of surfactant/stabilizer molecules influences not only nanoparticle dispersion but also surface chemical properties and hydrogen sorption properties. Polymers can also serve as surfactants, and their presence can accelerate hydrogen sorption.

The polymer matrix plays a critical role in determining the dispersion of nanoparticles and the optical and gas transport properties of the nanocomposite. Factors like polymer repeat unit, molecular weight, and processing parameters impact the nanoparticle distribution, optical transparency, and gas diffusivity. The polymer matrix can also function as a molecular filter, selectively allowing certain gases to enter, such as H2, while blocking others like CO.

The authors’ initial focus lies in plasmonic hydrogen sensing, where they described the use of nanofabricated arrays of Pd nanoparticles for gas detection. Spectral shifts in the LSPR peak of these nanoparticles occur upon hydrogen absorption, enabling the measurement of optical pressure composition isotherms. The sensing mechanism involves the efficient absorption of hydrogen into Pd, followed by dissociation, diffusion, and subsequent phase transformations within the material. This approach has been highly effective for hydrogen sensing, although the presence of hysteresis has posed challenges for continuous monitoring. To lay the foundation for rational plasmonic plastic nanocomposite design, the authors conducted investigations into the role of the three key components using 2D nanofabricated model systems on surfaces.

The hysteresis observed in Pd-H systems was mitigated by alloying Pd with Au, Ag, or Cu. These alloy systems exhibited improved sensing metrics and hysteresis-free responses to H2. Notably, the Pd₇₀Au₃₀ alloy system showed a balance between optical contrast and linearity.

The choice of surfactant/stabilizer molecules significantly impacted hydrogen sorption kinetics. Cationic surfactants like CTAB, CTAC, and TOAB slowed down hydrogen sorption, while polymeric surfactants like PVP accelerated it. The presence of polymers on nanoparticle surfaces reduced activation barriers for hydrogen sorption and enhanced optical contrast.

The polymer matrix selection influenced gas transport properties, with amorphous polymers like PMMA offering high transparency and rapid gas diffusion. Different polymers exhibited varying levels of sensor response acceleration and filtering ability, enabling the optimization of sensor performance through the use of polymer multilayers.

The authors explored the creation of fully functional plasmonic plastic materials for additive manufacturing. Unpublished results showcased the synthesis of Au-nanosphere: PLA and Au-nanorod: PMMA plasmonic plastic nanocomposites. These materials were used to create intricate 3D-printed models, demonstrating the feasibility of using plasmonic plastics for additive manufacturing. Using PMMA as the matrix, the authors created a nanocomposite where the shape of Pd nanocrystals was preserved. These sensors exhibited a response characteristic of Pd upon exposure to H₂. Despite a slow response, the sensors demonstrated resilience to CO poisoning in synthetic air. To address the slow response issue, Teflon AF was chosen as the matrix material due to its larger fractional free volume (FFV). The increased FFV led to faster gas diffusion, resulting in a significantly improved sensor response time, approaching real-time hydrogen monitoring. These sensors also maintained their response in the presence of CO.

The study by Darmadi et al. represents a groundbreaking achievement in the field of plasmonic sensing by introducing the concept of plasmonic plastic nanocomposites. These materials, combining colloidal metal nanoparticles and polymer matrices, open up exciting possibilities for additive manufacturing of plasmonic devices with enhanced functionalities and gas sensing capabilities. The rational design and optimization of plasmonic plastic constituents, including nanoparticles, surfactant molecules, and polymer matrices, have provided critical insights into creating high-performance sensors. These findings can guide the development of plasmonic plastics for diverse applications, beyond hydrogen sensing, such as the detection of other gases and analytes. Furthermore, the successful additive manufacturing of 3D plasmonic objects using these materials demonstrates the scalability and versatility of plasmonic plastics. This research paves the way for the integration of plasmonic sensors into various industries, including environmental monitoring, industrial safety, and medical diagnostics. The potential societal impact of this work is substantial, as it offers a new paradigm for cost-effective, scalable, and high-performance plasmonic sensing technologies.