At the heart of any biological process, from DNA replication and enzymatic activity to cellular motion and transformations, are small mechanical cues and forces that help drive reactions and guide synthetic pathways. As research pushes the frontiers of medicine and bioengineering, tracking and quantifying molecular-level forces, displacements, and torques will be become a critical component to unraveling the mysteries of the cell and Nature’s marvelous ability to sustain life. Furthermore, measuring nanomechanical events or mechanical responses is essential for developing novel label-free diagnostic devices and analytical probes. Most molecular machines and biological systems operate in the piconewton range and in complex media which puts immediate constraints on instrumentation. Of the many nanomechanical detection platforms, atomic force microscopes (AFMs) and optical/magnetic traps have emerged as the work-horse instruments that can resolve and extract quantitative force information from biological systems. However, it remains difficult to access the intracellular environment with these analytical tools, or multiplex the force read-outs, as they have fairly large sizes and feedback mechanisms. Our goal is to develop a new breed of nanofiber optic devices that not only can measure ultrasmall displacements (Å level) and forces (< 1 pN) but have the capacity to be multiplexed far beyond current state-of-the-art nanomechanical instruments.

(left) Schematic of the fiber optic force sensor along with an SEM image of a Au nanoparticle on a SnO₂ nanofiber waveguide. (right) Intensity time-course of a single Au nanoparticle scattering in the near-field of a nanofiber waveguide. The nanoparticle is tethered to the waveguide via a single DNA strand and shows 1 Å distance sensitivity while the DNA is compressed or stretched under fluid flow.

(left) Nanoindentation curves of different molecular weight PEG coatings on a nanofiber waveguide. The TEM micrograph captures a nanofiber optic force transducer. (right) Modeled force sensitivity of the nanofiber optic force transducer for various PEG molecular weights as a function of inter-chain spacing.

To do this we have been investigating near-field light-matter interactions occurring within evanescent field of SnO₂ subwavelength optical waveguides. For example, when a plasmonic nanoparticle is embedded in the evanescent field of the waveguide, the particle behaves like an optical antenna and expels photons from the core. By tracking the optical signal in the far-field, distance-dependent optical signals can be tracked with high resolution. Compared to fluorescent nanoparticles, we have shown that the plasmonic nanoparticles display over an order of magnitude improvement in the distance sensitivity reaching a resolution of 1 Å which is comparable to the best optical traps. To convert this molecular ruler platform into a force transducer there must be a mechanical resistance placed in between the nanoparticle and the waveguide. This can come in the form of thin, compressible fiber claddings such as polyethylene glycol (PEG) or polyelectrolyte multilayers. We now have well established protocols for synthesizing these polymer coatings and characterizing their mechanical properties via AFM. By tuning the thickness and stiffness of these films, the sensitivity and dynamic range of the transducers can be programmed. In addition to the experimental work, we have developed mechanistic models to predict the performance of these nanomechanical transducers and help guide the synthetic efforts. Ongoing work now includes experimentally calibrating the performance of these transducers and applying them to intracellular and biomolecular studies.

Related references

1.     D.J. Sirbuly, M. Law, J.C. Johnson, J. Goldberger, R.J. Saykally, and P.D. Yang “Nanoribbon waveguides for subwavelength photonics integration” Science 305, 1269-1273 (2004).

2.     D.J. Sirbuly, M. Law, P. Pauzauskie, H.Q. Yan, A.V. Maslov, K. Knutsen, C.Z. Ning, R.J. Saykally, and P.D. Yang “Optical routing and sensing with nanowire assemblies” Proc. Natl. Acad. Sci. U.S.A. 102, 7800-7805 (2005).

3.     D.J. Sirbuly, N.O. Fischer, S.C. J. Huang, A.B. Artyukhin, J.B.H. Tok, O. Bakajin and A. Noy “Biofunctional subwavelength optical waveguides for biodetection” ACS Nano 2, 255-262 (2008).

4.     I. Yoon, K. Kim, S. E. Baker, D. Heineck, S. C. Esener and D. J. Sirbuly “Stimulus-responsive light coupling and modulation with nanofiber waveguide junctions” Nano Lett. 12, 1905-1911 (2012).

5.     I. Yoon, S. E. Baker, K. Kim, Y. Wang, S. C. Esener and D. J. Sirbuly ” Profiling the evanescent field of nanofiber waveguides using self-assembled polymer coatings” Nanoscale 5, 552-555 (2013).

6.     I. Yoon, S. E. Baker, K. Kim, N. O. Fischer, D. Heineck, Y. Wang, S. C. Esener and D. J. Sirbuly “Nanofiber near-field light-matter interactions for enhanced detection of molecular level displacements and dynamics” Nano Lett. 13, 1440-1445 (2013).

7.     Q. Huang, I. Yoon, J. Villanueva, K. Kim and D. J. Sirbuly “Quantitative mechanical analysis of thin compressible polymer monolayers on oxide surfaces” Soft Matter 10, 8001-8010 (2014).

8.     J. Villanueva, Q. Huang and D. J. Sirbuly “Identification and design of novel polymeric nanomechanical transducers: A nano-structural model for thin film indentation” J. Appl. Phys.116, 104307 (2014).

Piezoelectric materials offer the most direct way of converting mechanical energy into an electrical potential or vice versa. Applications that utilize these effects are far reaching, ranging from loud speakers and acoustic imaging to energy harvesting and electrical actuators. However, piezoelectrics are intrinsically low power density materials and it has been difficult to boost the performance of these materials. It has been shown that reducing the dimensionality of the material can slightly enhance the piezoelectric coefficient, but major breakthroughs in the efficiency of these materials are required if they are to compete with higher power output and more sensitive electronic/photonic devices. We are therefore interested in understanding if the efficiency of piezoelectric materials is fundamentally limited or engineering limited. 

To help address this we have been investigating various ways to make new composite piezoelectric materials and interface the piezoelectric to different materials in order to enhance the energy transfer process or capture energy from non-mechanical energy sources (e.g., light, chemical, heat, etc.). For example, we have developed organic/inorganic hybrid energy-harvesting platforms by embedding piezoelectric nanowire arrays in an environment-responsive polymer matrix. Energy sources such as heat can then be used to swell the polymer which places a tensile load on the nanowires and produces a dc electric output from the nanowire array. Using this configuration we can achieve power densities of 20 nw/cm² at a temperature of only 65°C. These figures can be boosted by over 2-fold by simply altering the nanowire/polymer interface via non-slip adhesion promoters. 

(top left) Schematic of a hybrid piezoelectric nanowire-polymer energy nanoconverter. (bottom left) Image of ZnO nanowires embedded in a polymer matrix. (right) Electric current and power output of a nanowire-polymer energy nanoconverter under thermal (~ 65°C) stimulation.

 In addition to leveraging nanowire structures, we have been investigating piezoelectric polymer composites that interface piezoelectric nanoparticles with a polymer matrix. Due to the processability and biocompatibility of the polymer systems, there is a tremendous interest in fabricating high-efficiency piezoelectric polymers. Furthermore, these materials are much more amenable to 3D structuring which is extremely difficult to do with conventional electroceramics such as lead zirconate titanate (PbZr_xTi_1-xO₃, PZT) or barium titanate (BaTiO₃, BTO) and should have a significant impact on the development of biodiagnostics, imaging technologies, and nano/microelectromechanical systems. To boost the performance of piezoelectric polymers and provide a means of creating 2D and 3D shapes, we have been developing rapid printing strategies which allow the user to define the size, shape, and composition of the piezoelectric material. For example, we have utilized digital projection printing (DPP) to fabricate piezoelectric structures in mere seconds by combining BTO nanoparticles with a photoliable polymer solution. By grafting photosensitive chemical groups on the BTO nanoparticles, the piezoelectric crystals crosslink with the polymer chains under light exposure and enhance the mechanical-to-electrical energy conversion process by efficiently funneling the stress in the polymer chains to the piezoelectric nanoparticles. Ongoing work includes optimizing the printing process, understanding the chemical interface between the piezoelectric nanoparticles and the polymer matrix, and incorporating these materials into novel sensor and imaging platforms.

Related references

1.     X.Y. Wang, K. Kim, Y.M. Wang, M. Stadermann, A. Noy, A.V. Hamza, J.H. Yang, and D.J. Sirbuly "Matrix-assisted energy conversion in nanostructured piezoelectric arrays" Nano Lett.10, 4901-4907 (2010).

2.     K. Kim and D.J. Sirbuly, "Enhanced output of nanostructured piezoelectric arrays via controlled matrix/transducer interfacial interactions" Appl. Phys. Lett. 101, 213114 (2012).

3.     K. Kim, W. Zhu, X. Qu, C. Aaronson, W.R. McCall, S. Chen, and D.J. Sirbuly, "3D Optical Printing of Piezoelectric Nanoparticle-Polymer Composite Materials" ACS Nano, 8, 9799-9806 (2014).

4.     W.R. McCall, K. Kim, C. Heath, G. La Pierre, and D.J. Sirbuly, "Piezoelectric nanoparticle-polymer composite foams" ACS Appl. Mater. Interfaces, DOI: 10.1021/am506415y (2014).

The ability to guide and localize photons on features much smaller than the wavelength of light is critical for the development of new photonic chips, enhanced energy conversion schemes, and biosensors. Plasmonics leverages the strong light-matter interactions between materials with large free-electron densities and electromagnetic waves to confine light in subwavelength features and generate enhanced EM fields near the surface of the material. Most plasmonic materials are based on noble metals such as gold and silver which have plasmon resonances in the visible. However, there is growing demand for plasmonic activity in the infrared (IR) which eliminates the use of noble metals since their losses are extremely high at these wavelengths. Alternate materials for IR plasmonics are highly-doped semiconductors such as ZnO, GaAs, indium tin oxide (ITO), and silicon. We are interested in developing materials that not only have low loss in the IR, but also have highly tunable plasmonic resonances and can be fabricated with techniques that are compatible with complementary metal-oxide-semiconductor (CMOS) processing. For example, we have demonstrated that aluminum doped ZnO (AZO) can be deposited via atomic layer deposition (ALD) onto nanostructures with excellent conformality and with tunable plasmon resonances from the mid-IR to near IR. Ongoing work is focused on optimizing the properties of these materials and integrating them into energy conversion and other optoelectronic devices.

(left) Electron micrograph of a silicon nanopillar array coated (via ALD) with AZO. (middle) Optical density of ALD AZO thin films deposited at different temperatures. (right) Tunable plasmon resonances of AZO by altering the amount of aluminum.

 
Related references

1.     C.T. Riley, T.A. Kieu, J.S.T. Smalley, S.H.A. Pan, S.J. Kim, K.W. Post, A. Kargar, D.N. Basov, X. Pan, Y. Fainman, D. Wang, and D.J. Sirbuly "Plasmonic tuning of aluminum doped zinc oxide nanostructures by atomic layer deposition" Phys. Status Solidi RRL, DOI: 10.1002/pssr.201409359 (2014).

Awards:Our work on 3D optical printing of piezoelectric nanoparticle-polymer composites was chosen for the 2015 SPIE Best Paper Award in 3D printing, fabrication, and manufacturing.