BERNICY FONG
Rapid advances in technology have made single-photon-counting modules (SPCMs) useful in a broad range of new applications. Until recently, SPCMs were used mostly in controlled environments, almost exclusively in labs and research and development settings. Today, SPCMs are being integrated into OEM devices for many practical applications. They are being used to detect and measure materials such as environmental toxins, proteins, particles in emulsions and solutions, and for drug development, cancer and genetic research, and clinical lab diagnostics.
At the same time, advances in healthcare are accelerating the need for fast and affordable point-of-care (POC) solutions, stimulating development of new analytical and biomedical innovations. The sheer volume of POC markets is creating a growing demand for new instruments to provide better, more-accessible diagnostics. As a result, increasingly sophisticated POC diagnostic and analytical instrumentation are driving demand for higher-volume manufacturing, as well as the integration and deployment of highly complex devices, including SPCMs.
The information required for diagnostic applications requires fast, efficient detection of multiple photon parameters such as fluorescence intensity, diffusions, lifetimes, and polarizations. These parameters are molecular signatures that distinguish specific genes or protein characteristics. SPCMs enable quick and efficient collection and measurement of these and other factors.
SPCM technologies
SPCMs are ultrasensitive detectors that can identify and count single photons. These detectors can be based on photomultiplier tubes (PMTs), avalanche photodiodes (APDs), silicon photomultipliers (SiPMs), or superconducting nanowires. The ideal SPCM would be inexpensive, with high photon-detection efficiency (PDE) across the full light spectrum from UV to near-infrared (near-IR), and with zero dark count ("noise"), after-pulsing, and dead time.
In reality, tradeoffs between detector disposition and module design make this ideal combination of capabilities unachievable. Each type has its drawbacks: PMT-based SPCMs tend to provide higher PDE in the UV spectrum, but fall short in the near-IR. APD-based SPCMs produce higher dark count for the same active area as than other detector-based devices. SiPM photon counting modules have high after-pulse compared to other types of SPCMs. Superconducting nanowire photon counting detectors are very expensive.
Silicon-APD-based SPCMs incorporate the best combination of capabilities with minimal compromises in performance. With a small active area of about 150 μm in diameter, their dark count can be as low as <10 counts/s. This type of module responds to wavelengths from UV to near-IR with good signal-to-noise performance. It also has an after-pulsing probability of much less than 1%, with very short dead time for quick response, and is relatively cost-effective.
For photon counting, a special APD called a single-photon avalanche diode (SPAD; also called a Geiger mode APD or GMAPD) is operated in "Geiger mode." In Geiger mode, the APD is biased at much higher than the breakdown voltage. By limiting the breakdown current, the APD can remain in a stable "off" condition above breakdown without being damaged. When a photon-generated carrier is released in the multiplication region of the SPAD, impact ionization can occur in which electrons and holes are released and multiplied. Single-photon absorption can create an avalanche breakdown that can result in the generation of as many as 106 electrons.
A quenching resistor in series with the Geiger-mode APD is required to prevent the APD from being damaged by the avalanche current. As the voltage increases across the resistor during breakdown, the voltage in the multiplication region of the APD is reduced and the APD is eventually quenched. This is called "passive quenching" of a SPAD. Once the APD is quenched or reset, the diode is ready to be operated again at above breakdown voltage for subsequent photon detection. The disadvantage of this passive-quenching circuit is the longer dead time before the diode is available for the next photon detection. Active quenching can be used, which uses high-speed transistor switches to quench and reset the APD, allowing for much-shorter dead times and higher counting rates.
In general, a Geiger-mode APD needs temperature stabilization to work effectively. Thus, the photodiode is thermoelectrically cooled and temperature-controlled, ensuring consistent performance and optimal signal-to-noise ratio despite ambient temperature changes. Most SPCM electronics give output pulses in the form of transistor-to-transistor logic (TTL) pulses for photons detected for ease of integration into laboratory instrumentation.
Healthcare applications
In many emerging healthcare, bioimaging, and life-science applications, detection of extremely low optical signals and fluorescence in the 650–900 nm wavelength region is needed. This requires highly sensitive detection technologies, including SPCMs with high PDE in the red to near-IR range.
These applications also require SPCMs with high data rate, high resolution, low dead time (<20 ns), and fast counting rate. A very fast active quenching circuit with short dead time and short pulse will allow an SPCM to produce a fast count rate to meet this type of application requirement.
Photon-correlation spectroscopy is applied in protein, nanoparticle, and molecular analysis in the life-science arena. In this measurement technology, the detector's low after-pulsing probability is critical for attaining accurate results. After-pulse is caused by impurities within the crystal structure of the APD chip. After-pulse probability indicates the likelihood of a subsequent avalanche triggered by trapped high-energy electrons from the previous avalanche. Various steps in the APD wafer-growth process, such as dosage level and diffusion temperature, can contribute to the after-pulse, which can degrade the detector's sensitivity and render it ineffective for the intended use.
PDE is one of the most-important parameters of photon detector performance, as it reflects the detector's ability to translate single incoming photons into an actual electronic output signal. The Geiger-mode APD's chip structure defines how efficiently the chip can absorb the incoming photons and convert them into electrons. When a low k-factor (electron-hole ionization ratio) Geiger-mode APD is combined with a wide photon absorption region (>40 μm), it will optimize photon absorption in the 600–950 nm range and minimize noise generation. This enables the SPCM to detect extremely low fluorescence or optical signals in the red- to near-IR region, which are difficult to achieve with other detection technologies.
New electronic-component designs and printed circuit board (PCB) innovations have helped advance photon-counting modules from a passive quench to active quench circuitry, and evolve from low bandwidth (only a few-megacounts-per-second count rate) and high after-pulsing to today's more demanding performance levels.
Detecting biomarkers at low levels
Single-molecule counting (SMC), a branch of single-photon counting technologies, is enabling new diagnostic tools with greater levels of sensitivity for detecting small polypeptides in proteins.
SMC technology is 100 times more sensitive than contemporary immunoassay platforms, enabling high sensitivity and precise detection of low-abundance biomarkers. This technology can be applied to existing and newly identified biomarkers to aid physicians' decision-making across multiple disease areas in both acute and chronic disease management.
Cardiac care
For example, SMC technology is being used in advanced testing and management systems for cardiovascular disease. The cardiac protein, troponin, is present in healthy humans at extremely low levels that cannot be detected by traditional methods, limiting physicians' ability to clinically diagnose chronic cardiovascular disease. Previously available photon counting detectors did not have sufficiently high red/near-IR PDE to see the fluorescence emitting from this protein molecule in this specific wavelength range.
A high red/near-IR PDE silicon-APD-based photon-counting detector can now enable doctors to measure extremely low levels of troponin in seemingly healthy individuals—much lower levels than previously measurable, allowing unprecedented assessment of a patient's risk for cardiovascular disease.
Blood screening with dynamic light scattering
Single-photon counting also enables dynamic light scattering (photon-correlation spectroscopy) to achieve nanoscale particle detection and counting. New noninvasive, fast, accurate, and easy-to-use screening systems for blood and blood products are based on single-photon-correlation spectroscopy. Platelets perform vital functions in hemostasis and inflammation. The quality of blood platelets is critical for blood transfusion. Traditional methods for determining blood quality, used routinely before transfusion, are time-consuming and lack the detailed transparencies of platelet quality that includes fragmentation into microparticles.
Dynamic light scattering with single-photon counting enables analysis of platelets' size, concentration, and distribution of microparticles, so that their lifetime can be measured quickly and accurately (see figure). An SPCM module with PDE in the red wavelength range, high count rate, and low noise supports superior system performance. The results allow physicians, surgeons, and clinicians to determine the most suitable blood for the patients they are treating, improving patient safety in trauma and surgeries as well as optimizing cancer-treatment benefits.
Analyzing deep brain-tissue health
Another point-of-care analytical instrument that improves quality of life is a diffuse-correlation-spectroscopy, single-photon-counting-detector-based neuromonitor. This system provides noninvasive monitoring of blood flow in deep brain tissue in cases such as brain trauma, benign and malignant cerebral tumor differentiation, and premature infants' brain development.
Blood flow is associated with brain tissue health and function. The flow is measured by monitoring the tiny variations of photons induced by moving scatters in the brain's red blood cells. The photons emitted are in the red wavelength range. Traditional methods such as PET, SPECT, XeCT, PCT, MRI, and OCT are cumbersome, costly, invasive, and/or limited to analyzing the very top surface of brain tissue. A compact, cost-effective SPCM with high red/near-IR PDE can detect a small variation of photon emission, which allows for a highly sensitive and truly bedside portable system for deep tissue blood-flow monitoring. With accurate measurement and monitoring, physicians are able to implement timely and suitable treatment approaches.
Manufacturing challenges
SPCMs are self-contained modules that meet the low-light-level analytical detection demands of single-molecule detection and counting. In addition to POC applications, the modules are also widely used in superresolution microscopy, fluorescence, particle sizing, single-photon 3D lidar imaging, and nanotechnology. SPCMs are the key behind many nanotechnologies in the human health field, enabling scientific and diagnostic instrument developers to offer quick and affordable human-health solutions.
The temperature-sensitive nature of a Geiger-mode APD requires precise and constant temperature stabilization of the chip itself to keep the dark count low, and to ensure accurate photon counts. It can be difficult to reduce the module's size and minimize its cost while providing necessary thermal management to safeguard performance in analytical system environments. Thermoelectric coolers (TECs) can be costly, cumbersome, and consume significant power. And temperature-compensation circuits do not work well with APDs in Geiger mode because of the sensitivity of the bulk noise to temperature variation.
Various assembly and testing processes must be carefully streamlined to drive down the cost for a user-friendly module without compromising on quality and uniformity. The overall footprint of a module often needs to fit tight space requirements of the POC instruments, which should be reasonably portable for use in clinical instrumentation.
Helping to lower the cost of healthcare
Development of increasingly sensitive single-photon-counting modules is a springboard for technological innovations. These advances are enabling healthcare providers to detect increasingly lower levels of disease indicators faster, more economically, and with greater accuracy than ever before. In turn, this has fueled development of better, smaller, and more affordable analytical instruments that can be acquired by smaller POC facilities with limited space and budgets. The end result of all this progress is helping people gain access to better healthcare with earlier disease detection while helping to control the cost of care.
Bernicy Fong is product manager, low-light-level detection solutions at Excelitas Technologies, Waltham, MA; e-mail: [email protected]; www.excelitas.com.