Spectroscopy for Medicine—From SERS, Laser-Induced Fluorescence, and Biosensors to Analytical Nanotools

Working at the frontiers of biotechnology, fiberoptics, lasers technique and molecular spectroscopy, Tuan Vo-Dinh of Duke University has developed multiple sensor technologies useful for medical research and diagnostics. His contributions include the first antibody-based fiberoptics fluoroimmunosensor (FIS), the first antibody-based nanoprobe for monitoring biochemical species in a single living human cell, the first paper on the analytical use of surface-enhanced Raman spectroscopy (SERS) for chemical analysis, the development of a laser-induced fluorescence (LIF) technique for non-invasive optical biopsy, and plasmonic nanoprobe assays for early cancer diagnostics. Throughout this work Vo-Dinh and his research colleagues have brought spectroscopy to biomedical applications. In this interview, Vo-Dinh talks about his work in spectroscopy and photonics. Vo-Dinh is the winner of the 2019 Royal Society of Chemistry (UK) Sir George Stokes Award and gave a plenary lecture at the 2020 SciX conference in October, in Sparks, Nevada. This interview is part of an ongoing series with the winners of awards that are presented at SciX.

For many years you have worked in the development of analytical capabilities for antibody-based fiberoptics biosensor probes using the concept of the fluoroimmuno-sensor (FIS) (1). In early work on FIS you had proposed that these sensing probes would prove useful in detection of various analytes for cancer research, environmental monitoring, and other health applications. How did you become interested in this specific analytical approach and how have these tools proved effective or useful in clinical applications?

Throughout my research career, I have always been interested in working at the intersection of multiple scientific disciplines. I strongly believe that the integration of different areas of science and technology is needed to provide solutions to complex problems of global importance. More than three decades ago, I collaborated with a team of chemists, spectroscopists, and biologists from Oak Ridge National Laboratory (ORNL) and the University of Tennessee to integrate molecular spectroscopy, fiberoptics technology, laser techniques, and molecular biology; together we developed the first antibody-based fiberoptics fluoroimmunosensor (FIS) device. The FIS is a fiberoptics biosensing device that combines antibodies designed to recognize and trap target molecules of interest, laser excitation to induce fluorescence emission from the targets, and ultrasensitive optical detection. The FIS concept was quite a “novelty item” at that time. This technology has now opened new possibilities for a wide spectrum of chemical, environmental, and biomedical clinical applications, such as the assessment of an individual's exposure to chemical carcinogens, monitoring response to drug therapy, and identification of toxic substances in the environment. For instance, we demonstrated that the FIS is capable of detecting DNA-adduct biomarkers of human exposure to carcinogens in human clinical samples. The FIS technology opened new horizons for a fundamental technology of the "smart catheter/sensor" for in vivo biomedical analysis. Today the usefulness of fluoroimmuno-biosensors as  powerful bioanalytical tools is well recognized worldwide.

In the 1990s, we expanded the biosensing concept in a single-fiber approach to a multi-sensing platform– the multi-functional biochip–by combining microelectronics, molecular biology, and biotechnology. The device was one of the first chip-based 2D array biosensor systems combining molecular bioreceptors with CMOS phototransistors integrated circuits. It was capable of performing simultaneous detection of multiple disease end-points on a single platform by using different bioreceptors such as antibody and gene probes. The biochip could be used to diagnose genetic susceptibility and diseases, monitor exposure to bioactive environmental samples, and provide an important tool for medical diagnostics at the point-of-care and homeland defense applications.

You have been involved in a clinical study with the development of  a laser-induced fluorescence (LIF) technique for non-invasive optical biopsy (2). How is this approach different from the traditional biopsy procedures currently used for cancer diagnostics? Would you discuss this technology and its potential applications?

In-situ, rapid, and minimally invasive procedures for tissue diagnosis are important for early detection and timely treatment of cancer.  Traditionally, endoscopy is used to detect abnormal tissues in the esophagus.  Once an abnormality is found, biopsies are taken for determination of histopathology. The laboratory results are generally not available for several days.  There was an urgent need for a more practical technique that would allow real-time in vivo classification of the tissue type and provide a less invasive tool for cancer diagnosis at lower cost. In this clinical work, I collaborated with clinical colleagues at the Thompson Cancer Survival Center, in Knoxville, Tennessee, to develop a laser-induced fluorescence (LIF) technique that rapidly provided effective indices to diagnose cancer in the esophagus. LIF measurements were conducted during routine gastrointestinal endoscopic examinations of patients using a fiberoptic probe inserted into the biopsy channel of an endoscope. This LIF diagnostic information could be available in real time without the need of surgical biopsy. The LIF measurement was completed in approximately 0.6 second for each tissue site.  The results of this optical approach were compared with histopathology results of the biopsy samples and indicated excellent agreement in the classification of normal tissue and malignant tumors. The LIF method was successfully tested with over 100 patients with nearly 100% accuracy. The LIF method could lead to the development of a rapid, minimally invasive and cost‑effective technique for cancer diagnosis. Such a technique that is minimally invasive, requires no physical biopsy, and provides instantaneous results could revolutionize cancer diagnostics as we know it.

You have also worked for many years on the development and improvement of SERS (3,4). What were some of the most valuable or surprising discoveries that you encountered during your research in SERS? Has this technique proven to be a useful tool for trace organic analysis and other applications? What comments would you have regarding the analytical figures of merit and limitations of the SERS technique?

The SERS effect was discovered in 1977 by R. Van Duyne, J.A. Creighton, and coworkers, who reported an enormous intensity enhancement of Raman signals for some chemicals adsorbed on roughened electrode surfaces. In 1984, my team reported  the first demonstration of the analytical potential of the SERS effect. Until our work, the reproducibility and quantification of SERS-active media (mainly electrodes, silver sols) had been a major difficulty for SERS to be used in chemical analysis. An additional limitation in the early development of SERS was that the Raman enhancement had been observed for only a few highly polarizable small molecules, such as pyridine, benzoic acid, and their derivatives. Even the general applicability of the SERS effect was still questioned in the early 1980s. Our work on SERS analysis of several polycyclic compounds contributed to firmly establish the general applicability of the SERS phenomenon. We introduced a new type of SERS-active substrate based on a silver metal film on arrays of 30-nm nanoparticles. In a sense, more than three decades ago and long before the hype about nanotechnology, we were already adopting “nano” when nano wasn’t “cool’. The kind of SERS-active substrates we developed, also referred to as nanowaves or metal film on nanoparticles (MFON), has led to substantially improved reproducibility and quantification. Since then they have been widely adopted and used by other research groups.

My lab has also developed a wide variety of other types of SERS-active platforms, such as nanorods, nanodots, nanowires, nanostars and nanorattles. Our SERS nanoplatforms have multiple applications in environmental sensing, bioenergy research, biomedical diagnostics, and molecular imaging. In 1994 we introduced a novel type of nonradioactive SERS-based DNA probe that could be used for genomic analysis, biomedical diagnostics, and pathogen detection. A decade later, we developed a novel type of SERS-based nanoprobe, referred to as molecular sentinels (MS) and inverse molecular sentinels (iMS) that can be used for the detection of early biomarkers of diseases. The iMS homogeneous assay format requires no washing steps and no sample amplification due to the intense signal amplification of SERS. We used the iMS  assay to monitor  a class of molecular targets known as microRNAs. MicroRNAs have demonstrated great promise as important biomarkers for early detection of various cancers, yet these small molecules have not been adopted into early diagnostics for clinical practice because of challenging analytical aspects in the lab. Microarray-based technology, northern blotting, sequencing, and quantitative real time polymerase chain reaction (qRT-PCR) analyses are often employed, but these lab-based techniques involve elaborate, time-consuming, and expensive equipment. Our team demonstrated successful detection of microRNA biomarkers for esophageal adenocarcinoma within patient tissue samples without requiring sample amplification (no PCR needed). Our studies have demonstrated the potential of the SERS-based nanoprobes for the detection of cancers (gastrointestinal  cancer, and head and neck cancer) and infectious diseases (HIV, dengue, malaria). Our SERS nanoprobe approach could lead to an accessible strategy for early diagnosis, which is a major unmet need in screening and will have transformative potential for medical research as well as diagnostic applications at the point of care.

I am also fascinated by the contribution of science and technology to the sustainability of our world. In this line of research, I worked with a team of bioengineers and plant biologists at Duke and physicists at Argonne National Laboratory to further develop the SERS iMS biosensing technology for renewable bioenergy research. Knowing when and which genes turn on inside a plant would be enormously useful to scientists because they could potentially select plants with preferred growth patterns or control plant growth for optimizing biofuel production. We  used our technology for sensing and imaging specific microRNAs that regulate flowering and vegetative growth in plants. We demonstrated the possibility to detect and image genomic biomarkers in living plant leaves under ambient conditions without requiring sample extraction and lab-based analysis. This research sets the basis for functional in vivo imaging of genomic biomarkers in plants, a much needed tool for in vivo plant biology research and biofuel development.

In your work you have synthesized gold nanostars for use in SERS (5). Are you able to tune these nanostars for specific applications? What is the theory and modeling approach that can be used to tune these? For example, you have reported that variations in star size resulted in observed shifts of the long plasmon band in the near-infrared (NIR) region.

Among various kinds of metallic nanoparticles, gold nanostars (GNS) are of particular interest as they offer optical tunability by engineering subtle changes in their geometry. Incident light (for example, a laser beam) irradiating metallic nanoparticles or nanostructures induces oscillations in  the metal conduction electrons. These oscillating electrons, which are called surface plasmons-hence the term plasmonic nanoparticles, produce secondary intense electric fields concentrated at high curvature points on nanoparticles. The multiple sharp branches of GNS are ideal structures to create the so-called “lightning rod” effect that enhances the local electromagnetic field dramatically, thus producing intense SERS signals from molecules that are on or near the branch tips, in very good agreement with our theoretical calculations. Recognizing this unique property, my lab first proposed GNS as a platform for SERS sensing.

  With the goal of developing biocompatible nanoparticles for in vivo biomedical applications, we first introduced a new surfactant-free synthesis of GNS that does not require toxic surfactant chemicals often used in nanoparticle synthesis. We demonstrated that GNS optical properties could be engineered by making subtle changes in their synthesis chemistry that resulted in geometry changes. For biomedical applications involving in vivo excitation in deep tissue, we demonstrated that an increase in the branch number of GNS can shift their peak absorption band into the near-infrared (NIR) region (700–1200 nm). This spectral range is very important for in vivo medical applications and is often called the tissue optical window, where photons are less absorbed by tissue and can travel further in tissue for sensing and therapy.

We can also control the size of GNS such that they can passively accumulate in tumors selectively by taking advantage of the enhanced permeability and retention (EPR) effect of tumor vasculature for cancer therapy. The EPR effect is a result of the inherent leakiness of the tumor vasculature, which allows nanoparticles having certain sizes to escape the circulation and accumulate passively in tumors. In addition, retention of nanoparticles in the tumor is enhanced by the lack of an efficient lymphatic system that would normally carry extravasated fluid back to the circulatory system.

For light-induced photothermal therapy applications we showed that GNS are excellent nano-sources for heating tumor cells ”from the inside.” In hyperthermia treatment of tumors, elevating the temperature to more than 42 °C, malignant cells are killed through apoptosis or necrosis. Traditional hyperthermia modalities, such as microwave, or radiofrequency, and focused ultrasound have been used to control macroscopic heating around the tumor region, but cannot target or ablate cancer cells at the microprecision scale. GNS, which accumulate preferentially in and around cancer cells, can be triggered with light to rapidly achieve high ablative intratumoral temperatures and can also induce milder fever-range hyperthermia in the tumor microenvironment. We demonstrated that GNS-mediated treatment with a near-infrared laser provides a controllable method to photothermally treat sarcoma in the murine tumor model. We also showed that, in a brain cancer mouse model, it was possible to achieve selective accumulation of GNS within target brain tumors but not in adjacent normal brain tissue, demonstrating spatial specificity in focal disruption of brain tumor vasculature and the blood-brain barrier for potential drug delivery applications. The combination of the EPR effect and the capacity for efficient photon to heat conversion make GNS an ideal photothermal transducer for selective cancer therapy at the cellular level.

Your research in 2000 described an antibody-based nanoprobe for in situ measurements of a single cell (6). The nanoprobe you selected used antibody-based receptors targeted to a fluorescent analyte, benzopyrene tetrol (BPT). You had hoped that the detection of this BPT nanoprobe could be used as a biomarker for monitoring DNA damage due to exposure to the carcinogen benzo[a]pyrene (BaP) and that this may result in a technique for possible precancer diagnosis. Would you tell our readers how these types of probes have been applied and what clinical applications or additional research has resulted from this work?

Two decades ago, we developed the first antibody-based nanosensor capable of monitoring chemicals in a single living cell. The nanosensors had tiny probes fabricated with optical fibers pulled down to tips with distal ends of approximately 30 nm. The nano-scale size of this new class of sensors allows for measurements in the smallest of environments.  One such environment that has evoked a great deal of interest is that of individual living cells.  Using these nano-biosensors, it has become possible to probe individual chemical species in specific locations throughout a cell. The fiberoptics nanoprobes were covalently bound with antibodies designed to target benzopyrene tetrol (BPT), a  biomarker of DNA-adducts of the carcinogen benzo[a]pyrene (BaP). DNA adduct formation involves the binding of a compound to DNA, a process that could damage DNA, resulting in abnormal replication and, without proper DNA repair, could lead to a cancerous cell. Monitoring DNA adducts is a strategy that could be used to monitor the extent of environmental exposure and health effects on people before occurrence of cancer. Such a precancer diagnosis approach could lead to appropriate pretreatment or early prevention actions. We focused on the detection of BPT because it is a biomarker of human exposure to BaP, a byproduct of combustion processes and a compound of great environmental and toxicological interest due to its mutagenic and carcinogenic properties and its ubiquitous presence in the environment.

We also used nanobiosensors to monitor a molecular signaling process, apoptosis, in a single living cell that was treated with a photodynamic drug for cancer. Apoptosis or programmed cell death is a process by which cells degenerate during normal development, aging, or disease. When the process of apoptosis malfunctions, it can lead to clinical problems such as cancer and neurodegenerative disease. Drugs are often designed to provoke apoptosis selectively in cancer cells. Unlike traditional bioanalytical techniques, the nanobiosensor technology could probe the cell machinery, elucidating life-and-death processes such as apoptosis. This will help us better understand and cure disease occurring at the cellular and molecular level that were heretofore invisible to human inquiry.

In your research you have introduced the concept of synergistic immune photothermal nanotherapy (SYMPHONY) for potential cancer treatment of inoperable cancer sites (7). Would you explain this approach and any further results or insights that have been gained from this work? How does your approach differ from other forms of photodynamic therapy?

From disease diagnostics, my research interests also expanded to the treatment of illnesses such as cancer. We have recently seen an increasing interest in immunotherapy. Molecular processes, called immune checkpoints, are normally safeguards used by the body to prevent inappropriate processes or control overactivation of the immune response. However, many tumors have acquired the ability to manipulate these checkpoints and to block the action of the immune system. Drugs designed to counter-block immune checkpoints have recently shown efficacy in the treatment of certain cancers. In this interdisciplinary study, I collaborated with a group of bioengineers, medical researchers, and clinicians throughout the Duke campus and the Duke medical school. Our team demonstrated that the use of GNS in combination with a checkpoint blockade immunotherapy drug (antiPD-L1)-a treatment we referred to as synergistic immuno photothermal nanotherapy (SYMPHONY)-can  dramatically enhance the efficacy of immunotherapy. Our studies indicated it is possible to achieve complete eradication of primary treated tumors as well as distant untreated tumors in mice implanted with a bladder cancer cell line. GNS were designed to efficiently absorb laser light for conversion into heat and to accumulate preferentially within a tumor due to the EPR effect. We now have the combined ability to use laser light in order to selectively and effectively treat tumor areas where GNS are located while keeping surrounding healthy tissues at significantly lower and safer temperatures. This GNS-mediated photothermal treatment strategy offers significant advantages over other traditional thermal therapies such as ultrasound, microwaves, radiofrequency, which can control macroscopic heating around the tumor region, but cannot target or ablate cancer cells at microprecision scale and cannot differentiate tumor cells from healthy cells. Our studies showed that the effectiveness of the combination therapy was synergistic and not just additive. Remarkably, we found that SYMPHONY not only eradicated primary tumors treated by the laser but also induced immune-mediated destruction of distant metastatic tumors that were untreated by the laser. The approach could provide an effective treatment when aggressive tumors that spread throughout the body cannot be surgically removed. It is our hope that the SYMPHONY strategy could lead to an entirely new treatment paradigm that challenges traditional surgical resection approaches for many cancers and metastases.

Of great interest was our observation that the SYMPHONY approach was capable of inducing long-term immunological memory that can provide protection against future tumor recurrence. In our studies involving two murine models of bladder cancer and brain tumor (glioblastoma), delayed rechallenge with repeated tumor injections did not lead to new tumor formation, indicating that the combined treatment induced effective long-lasting immunity that could provide protection against tumor recurrence long after treatment of the initial tumors, like an “anticancer vaccine” effect. We are in the initial stage of this SYMPHONY project and more studies involving larger cohorts of laboratory animals and different types of cancer cell lines are being performed. Further fundamental studies are also being conducted by our team to obtain better understanding of the mechanisms underlying these novel synergistic treatment modalities in order to help us enhance and broaden the effect of immune-checkpoint inhibitors for successful eradication of metastatic cancer.

What were some of the key challenges you have encountered during your career of laboratory research and teaching?

Life presents plenty of challenges to us. And challenges often create new opportunities. There are important challenges facing scientists today. With our planet’s limited resources, we are now witnessing a paradigm shift from a development-driven society (20^th century) to a sustainability-driven society (21^st century). Scientists and engineers in photonics like me, all have many opportunities to use our knowledge, learn new skills, and apply our innovativeness to address the global sustainability and environmental challenges, and ultimately contribute to a sustainable future as best as we can.

Another great challenge of our time is the exponential increase in the complexity of knowledge. In my teaching classes, I try to instill in my students a certain sense of scientific humility by telling them that no single discipline can solve big problems. I hope this statement will encourage my students to pursue teamwork and establish cross-disciplinary collaborations in their future careers. We are now entering a new phase where the knowledge of individual elements is no longer sufficient but should be combined and integrated in order to attain knowledge at the next level, that is at the multi-scale systems level where the information on organization, activity and function requires a much higher level of complexity and sophistication. This transition from a knowledge base of individual elements to a systems level is one of the major paradigm shifts of the 21^st century, which can be achieved only by integrating multiple disciplines and different domains of knowledge. Education and research in this and coming decades will evolve into a framework to fit the new reality of our world, a world that will be faced with and embrace cross-disciplinary, systems level, and global challenges. It is important for us to educate our students, the next generation of innovators and leaders, not only to solve scientific and technical problems but also to understand societal connections between various human activities, and create bridges between elements spanning multiple disciplines in order to ultimately build a better interconnected world.

What would you consider to be the most meaningful contributions of your work, including patents?

An invention or new technique is, in an intellectual sense, like a child that you have created and nurtured throughout your work and scientific career. Each has its own unique features and values. Therefore, I have no favorite invention or contribution; I cherish all of them. They all have a favorite place in my heart.

I feel very fortunate to have the opportunity to contribute in the important field of photonics. I believe that photonics is one of the ultimate enabling technologies that has played a crucial role by contributing key revolutionary and disruptive advances, and will open new avenues and new possibilities that will define the next century. Light influences our lives today in new ways that we could never have imagined just a decade ago. As we enter a new decade, light will play an even more significant role, further empowering the information revolution in global communications, creating new nanotools to unveil the inner world of matter, uncover medical cures to save us from emerging global health threats, inventing new renewable energy sources, and galvanizing human exploration to the frontiers of the universe.

Would you share with our readers to describe your work ethic,  philosophy, and how you plan your daily or weekly work schedule?

The career of a scientist requires a passion to learn and discover new things with the hope that the fruit of his or her efforts can contribute, even a very small part, to society. I would also say that it involves tremendous efforts in innovative thinking and intellectual pursuits. It also requires large amounts of energy for the long working hours required for inventing new ideas, designing new devices, planning new experiments, writing reports, publications, and research grants as well as many other professional duties. However, if you have a great passion for what you are doing, this is a small price to pay for a very intellectually rewarding life experience. Stimulating collaborative work, learning new knowledge, and exchanges of new ideas with colleagues from other disciplines are added benefits that I truly cherish. I have had the opportunity to work with many talented coworkers, postdocs, and students; they all share with me, as a team, all the recognitions and honors that I have received throughout my career.

What words of wisdom do you have for any young people interested in a scientific research career?

This is a most important and exciting time in science. Dream out of the box. Think long-term. Be patient.

References

1. T. Vo-Dinh, B.J. Tromberg, G.D. Griffin, K.R. Ambrose, M.J. Sepaniak, and E.M. Gardenhire, Antibody-based fiberoptics biosensor for the carcinogen benzo (a) pyrene, Appl. Spectrosc. 41(5), 735–738 (1987).
2. T. Vo-Dinh, M. Panjehpour and B.F. Overholt, Laser-Induced Differential Normalized Fluorescence Method for Cancer Diagnosis," US Patent 5,579,773 (1996).
3. T. Vo-Dinh, M.Y.K. Hiromoto, G.M. Begun, and R.L. Moody, Surface-enhanced Raman spectrometry for trace organic analysis, Anal. Chem. 56(9), 1667–1670 (1984).
4. T. Vo-Dinh, Surface-enhanced Raman spectroscopy using metallic nanostructures, TrAC-Trend. Anal. Chem. 17(8-9), 557–582 (1998).
5. C.G. Khoury and T. Vo-Dinh, Gold nanostars for surface-enhanced Raman scattering: synthesis, characterization and optimization, J. Phys. Chem.112(48), pp.18849–18859 (2008).
6. T. Vo-Dinh, J.P. Alarie, B.M. Cullum, G.D. and Griffin, Antibody-based nanoprobe for measurement of a fluorescent analyte in a single cell, Nat. Biotechnol. 18(7), 764–767 (2000).
7. Y. Liu, P.  Maccarini, G.M. Palmer, W. Etienne, Y. Zhao, C.T. Lee, X. Ma, B.A. Inman, and T. Vo-Dinh, Synergistic immuno photothermal nanotherapy (SYMPHONY) for the treatment of unresectable and metastatic cancers, Sci. Rep.7(1), 1–6  (2017).

Dr. Vo-Dinh is R. Eugene and Susie E. Goodson Distinguished Professor of Biomedical Engineering, Professor of Chemistry, and Director of the Fitzpatrick Institute for Photonics at Duke University. After high school in Vietnam, he pursued studies in Europe, receiving a B.S. in physics at EPFL-Lausanne (1970) and a Ph.D. in physical chemistry at ETH-Zurich (1975). Before joining Duke University in 2006, he was Director of the Center for Advanced Biomedical Photonics and a Corporate Fellow, one of the highest honors for distinguished scientists at Oak Ridge National Laboratory (ORNL). His main research goal is focused on developing advanced technologies to protect the environment and human health. His research has centered on the development, integration and application of biophotonics, molecular spectroscopy, molecular biology and nanotechnology for biomedical diagnostics, photoimmunotherapy, precision medicine, and global health. 

Dr. Vo-Dinh has received seven R&D 100 Awards for Most Significant Advance in Research and Development; the Gold Medal Award, Society for Applied Spectroscopy (1988); the Languedoc-Roussillon Award (France) (1989); the Scientist of the Year Award, ORNL (1992); the Thomas Jefferson Award, Martin Marietta Corporation (1992); two Awards for Excellence in Technology Transfer, Federal Laboratory Consortium (1995, 1986); the Lockheed Martin Technology Commercialization Award (1998); the Distinguished Inventors Award, UT-Battelle (2003); the Distinguished Scientist of the Year Award, ORNL (2003); the Exceptional Services Award, U.S. Department of Energy (1997); the Award for Spectrochemical Analysis, American Chemical Society (ACS) Division of Analytical Chemistry (2011); and the Sir George Stokes Award, Royal Society of Chemistry (United Kingdom (2019)). He has authored over 500 publications, is a Fellow of the U.S. National Academy of Inventors and holds over 50 patents.