Micromachined nanocalorimetric sensor for ultra-low-volume cell-based assays

Sub-nL thin-film differential scanning calorimetry chip for rapid thermal analysis of liquid samples

Differential scanning calorimetry (DSC) is a popular thermal analysis technique. The miniaturization of DSC on chip as thin-film DSC (tfDSC) has been pioneered for the analysis of ultrathin polymer films at temperature scan rates and sensitivities far superior to those attainable with DSC instruments. The adoption of tfDSC chips for the analysis of liquid samples is, however, confronted with various issues including sample evaporation due to the lack of sealed enclosures. Although the subsequent integration of enclosures has been demonstrated in various designs, rarely did those designs exceed the scan rates of DSC instruments mainly because of their bulky features and requirement for exterior heating. Here, we present a tfDSC chip featuring sub-nL thin-film enclosures integrated with resistance temperature detectors (RTDs) and heaters. The chip attains an unprecedented sensitivity of 11 V W-1 and a rapid time constant of 600 ms owing to its low-addenda design and residual heat conduction (∼6 μW K-1). We present results on the phase transition of common liquid crystals which we leverage to calibrate the RTDs and characterize the thermal lag with scan rates up to 900 °C min-1. We then present results on the heat denaturation of lysozyme at various pH values, concentrations, and scan rates. The chip can provide excess heat capacity peaks and enthalpy change steps without much alteration induced by the thermal lag at elevated scan rates up to 100 °C min-1, which is an order of magnitude faster than those of many chip counterparts.

A Micromachined Picocalorimeter Sensor for Liquid Samples with Application to Chemical Reactions and Biochemistry

Calorimetry has long been used to probe the physical state of a system by measuring the heat exchanged with the environment as a result of chemical reactions or phase transitions. Application of calorimetry to microscale biological samples, however, is hampered by insufficient sensitivity and the difficulty of handling liquid samples at this scale. Here, a micromachined calorimeter sensor that is capable of resolving picowatt levels of power is described. The sensor consists of low-noise thermopiles on a thin silicon nitride membrane that allow direct differential temperature measurements between a sample and four coplanar references, which significantly reduces thermal drift. The partial pressure of water in the ambient around the sample is maintained at saturation level using a small hydrogel-lined enclosure. The materials used in the sensor and its geometry are optimized to minimize the noise equivalent power generated by the sensor in response to the temperature field that develops around a typical sample. The experimental response of the sensor is characterized as a function of thermopile dimensions and sample volume, and its capability is demonstrated by measuring the heat dissipated during an enzymatically catalyzed biochemical reaction in a microliter-sized liquid droplet. The sensor offers particular promise for quantitative measurements on biological systems.

Fast antimicrobial susceptibility testing on Escherichia coli by metabolic heat nanocalorimetry

Fast spreading of antimicrobial resistance is now considered a major global health threat. New technologies are required, enabling rapid diagnostics of bacterial infection combined with fast antimicrobial susceptibility testing (AST) for evaluating the efficiency and dosage of antimicrobial compounds in vitro. This work presents an integrated chip-based isothermal nanocalorimetry platform for direct microbial metabolic heat measurements and evaluates its potential for fast AST. Direct detection of the bacteria-generated heat allows monitoring of metabolic activity and antimicrobial action at subinhibitory concentrations in real-time. The high heat sensitivity of the platform enables bacterial growth detection within only a few hours of incubation, whereas growth inhibition upon administration of antibiotics is revealed by a decrease or the absence of the heat signal. Antimicrobial stress results in lag phase extension and metabolic energy spilling. Oxygen consumption and optical density measurements provide a more holistic insight of the metabolic state and the evolution of bacterial biomass. As a proof-of-concept, a metabolic heat-based AST study on Escherichia coli as model organism with 3 clinically relevant antibiotics is performed and the minimum inhibitory concentrations are determined.

Sub-nanowatt microfluidic single-cell calorimetry

Non-invasive and label-free calorimetry could become a disruptive technique to study single cell metabolic heat production without altering the cell behavior, but it is currently limited by insufficient sensitivity. Here, we demonstrate microfluidic single-cell calorimetry with 0.2-nW sensitivity, representing more than ten-fold enhancement over previous record, which is enabled by (i) a low-noise thermometry platform with ultralow long-term (10-h) temperature noise (80 μK) and (ii) a microfluidic channel-in-vacuum design allowing cell flow and nutrient delivery while maintaining a low thermal conductance of 2.5 μW K⁻¹. Using Tetrahymena thermophila as an example, we demonstrate on-chip single-cell calorimetry measurement with metabolic heat rates ranging from 1 to 4 nW, which are found to correlate well with the cell size. Finally, we perform real-time monitoring of metabolic rate stimulation by introducing a mitochondrial uncoupling agent to the microchannel, enabling determination of the spare respiratory capacity of the cells.

Calorimetrically measuring the heat of single cells is currently not possible due to the sensitivity of existing calorimeters. Here the authors present on-chip single cell calorimetry, with a sensitivity over ten-fold greater than the current gold-standard.

The challenge of intracellular temperature

This short review begins with a brief introductory summary of luminescence nanothermometry. Current applications of luminescence nanothermometry are introduced in biological contexts. Then, theoretical bases of the “temperature” that luminescence nanothermometry determines are discussed. This argument is followed by the 10⁵ gap issue between simple calculation and the measurements reported in literatures. The gap issue is challenged by recent literatures reporting single-cell thermometry using non-luminescent probes, as well as a report that determines the thermal conductivity of a single lipid bilayer using luminescence nanothermometry. In the end, we argue if we can be optimistic about the solution of the 10⁵ gap issue.

Direct determination of anaerobe contributions to the energy metabolism of Trypanosoma cruzi by chip calorimetry

We describe a chip calorimetric technique that allows the investigation of biological material under anoxic conditions in a micro-scale and in real time. Due to the fast oxygen exchange through the sample flow channel wall, the oxygen concentration inside the samples could be switched between atmospheric oxygen partial pressure to an oxygen concentration of 0.5% within less than 2 h. Using this technique, anaerobic processes in the energy metabolism of Trypanosoma cruzi could be studied directly. The comparison of the calorimetric and respirometric response of T. cruzi cells to the treatment with the mitochondrial inhibitors oligomycin and antimycin A and the uncoupler FCCP revealed that the respiration-related heat rate is superimposed by strong anaerobic contributions. Calorimetric measurements under anoxic conditions and with glycolytic inhibitors showed that anaerobic metabolic processes contribute from 30 to 40% to the overall heat production rate. Similar basal and antimycin A heat rates with cells under anoxic conditions indicated that the glycolytic rates are independent of the oxygen concentration which confirms the absence of the "Pasteur effect" in Trypanosomes. Graphical abstract.

Difference in intracellular temperature rise between matured and precursor brown adipocytes in response to uncoupler and β-adrenergic agonist stimuli

Brown adipocytes function to maintain body temperature by heat production. However, direct measurement of heat production at a single cell level remains difficult. Here we developed a method to measure the temperature within primary cultured brown adipocytes using a cationic fluorescent polymeric thermometer. Placement of the thermometer within a matured brown adipocyte and a precursor cell enabled the detection of heat production following uncoupler treatment. The increase in the intracellular temperature due to stimulation with a mitochondrial uncoupler was higher in matured brown adipocytes than in precursor cells. Stimulation with a β-adrenergic receptor (β-AR) agonist, norepinephrine, raised the intracellular temperature of matured brown adipocytes to a level comparable to that observed after stimulation with a β3-AR-specific agonist, CL316.243. In contrast, neither β-AR agonist induced an intracellular temperature increase in precursor cells. Further, pretreatment of brown adipocytes with a β3-AR antagonist inhibited the norepinephrine-stimulated elevation of temperature. These results demonstrate that our novel method successfully determined the difference in intracellular temperature increase between matured brown adipocytes and precursor cells in response to stimulation by an uncoupler and β-AR agonists.

Thermodynamics of Bioreactions

Thermodynamic principles have been applied to enzyme-catalyzed reactions since the beginning of the 1930s in an attempt to understand metabolic pathways. Currently, thermodynamics is also applied to the design and analysis of biotechnological processes. The key thermodynamic quantity is the Gibbs energy of reaction, which must be negative for a reaction to occur spontaneously. However, the application of thermodynamic feasibility studies sometimes yields positive Gibbs energies of reaction even for reactions that are known to occur spontaneously, such as glycolysis. This article reviews the application of thermodynamics in enzyme-catalyzed reactions. It summarizes the basic thermodynamic relationships used for describing the Gibbs energy of reaction and also refers to the nonuniform application of these relationships in the literature. The review summarizes state-of-the-art approaches that describe the influence of temperature, pH, electrolytes, solvents, and concentrations of reacting agents on the Gibbs energy of reaction and, therefore, on the feasibility and yield of biological reactions.

Optical visualisation of thermogenesis in stimulated single-cell brown adipocytes

The identification of brown adipose deposits in adults has led to significant interest in targeting this metabolically active tissue for treatment of obesity and diabetes. Improved methods for the direct measurement of heat production as the signature function of brown adipocytes (BAs), particularly at the single cell level, would be of substantial benefit to these ongoing efforts. Here, we report the first application of a small molecule-type thermosensitive fluorescent dye, ERthermAC, to monitor thermogenesis in BAs derived from murine brown fat precursors and in human brown fat cells differentiated from human neck brown preadipocytes. ERthermAC accumulated in the endoplasmic reticulum of BAs and displayed a marked change in fluorescence intensity in response to adrenergic stimulation of cells, which corresponded to temperature change. ERthermAC fluorescence intensity profiles were congruent with mitochondrial depolarisation events visualised by the JC-1 probe. Moreover, the averaged fluorescence intensity changes across a population of cells correlated well with dynamic changes such as thermal power, oxygen consumption, and extracellular acidification rates. These findings suggest ERthermAC as a promising new tool for studying thermogenic function in brown adipocytes of both murine and human origins.

Multiscale Thermal Property Measurements for Biomedical Applications

Bioheat transfer-based innovations in health care include applications such as focal treatments for cancer and cardiovascular disease and the preservation of tissues and organs for transplantation. In these applications, the ability to preserve or destroy a biomaterial is directly dependent on its temperature history. Thus, thermal measurement and modeling are necessary to either avoid or induce the injury required. In this review paper, we will first define and discuss thermal conductivity and calorimetric measurements of biomaterials in the cryogenic (<-40 °C), subzero (<0 °C), hypothermic (<37 °C), and hyperthermic (>37 °C) regimes. For thermal conductivity measurements, we review the use of 3ω and laser flash techniques for measurement of thermal conductivity in thin (1 μm-2 mm thick), anisotropic, and/or multilayered tissues. At the nanoscale, we review the use of pump-probe and scanning probe methods to measure thermal conductivity at short temporal scales (10 ps-100 ns) and spatial scales (1 nm-1 μm), particularly in the coating and surrounding medium around metallic nanoparticles (1 nm-20 nm). For calorimetric techniques, we review differential scanning calorimetry (DSC), which is intrinsically at the microscale (e.g., tissue pieces or millions of cells in media). DSC is used with large sample mass (∼3-100 mg) over wide temperature ranges (-180 to 750 °C) with low-temperature scanning rates (<750 °C/min). The need to assess smaller samples at higher rates has led to the development of nanocalorimetry on a silicon based membrane. Here the sample weight is as low as 10 ng, thereby allowing ultra-rapid heating rates (∼1 × 10⁷ C/min). Finally, we discuss various opportunities that are driving the need for new micro- and nanoscale thermal measurements.

Direct organelle thermometry with fluorescence lifetime imaging microscopy in single myotubes

FLIM of ER thermo yellow and non-targeted mCherry reveals the Ca²⁺-dependent heat production localized to SR in C2C12 myotube.

Fabrication and characterization of a multichannel 3D thermopile for chip calorimeter applications

Thermal sensors based on thermopiles are some of the most robust and popular temperature sensing technologies across industries and research disciplines. A chip calorimeter with a 3D thermopile layout with a large sensing area and multichannel capacity has been developed, which is highly desired for many applications requiring large reaction chambers or high throughputs, such as biofilm research, drug screening, etc. The performance of the device, including temperature sensitivity and heat power sensitivity, was evaluated. The capability to split the chip calorimeter to multiple channels was also demonstrated, which makes the chip calorimeter very flexible and powerful in many applications.

Nanocalorimetric platform for accurate thermochemical studies in microliter volumes

We present a nanocalorimetric platform for accurate thermochemical studies of (bio-)chemical reactions in a miniaturized format, characterized by fast thermalization time, excellent base temperature stability and fast sensing response time.

Potentials and limitations of miniaturized calorimeters for bioprocess monitoring

In theory, heat production rates are very well suited for analysing and controlling bioprocesses on different scales from a few nanolitres up to many cubic metres. Any bioconversion is accompanied by a production (exothermic) or consumption (endothermic) of heat. The heat is tightly connected with the stoichiometry of the bioprocess via the law of Hess, and its rate is connected to the kinetics of the process. Heat signals provide real-time information of bioprocesses. The combination of heat measurements with respirometry is theoretically suited for the quantification of the coupling between catabolic and anabolic reactions. Heat measurements have also practical advantages. Unlike most other biochemical sensors, thermal transducers can be mounted in a protected way that prevents fouling, thereby minimizing response drifts. Finally, calorimetry works in optically opaque solutions and does not require labelling or reactants. It is surprising to see that despite all these advantages, calorimetry has rarely been applied to monitor and control bioprocesses with intact cells in the laboratory, industrial bioreactors or ecosystems. This review article analyses the reasons for this omission, discusses the additional information calorimetry can provide in comparison with respirometry and presents miniaturization as a potential way to overcome some inherent weaknesses of conventional calorimetry. It will be discussed for which sample types and scientific question miniaturized calorimeter can be advantageously applied. A few examples from different fields of microbiological and biotechnological research will illustrate the potentials and limitations of chip calorimetry. Finally, the future of chip calorimetry is addressed in an outlook.

Nanoscale thermal analysis for nanomedicine by nanocalorimetry

Microfabricated nanocalorimeters sensitively measure the thermal properties of nanomaterials and can be used for biomedical and in vitro measurements. This review examines the capabilities of nanocalorimeters including specific applications to nanomedicine such as measurements of nanomaterial stability, protein crystallization, ligand-protein binding, phase transitions, phase separations, interfacial reactions, and sorption-desorption phenomena. Widespread adoption of nanotechnology into clinical medicine will require a more complete understanding of the basic properties of nanomaterials, the relationship between nanomaterial processing, and physical properties and a deeper understanding of how nanomaterial physical properties control biological interactions. Nanocalorimetry is suitable where high sensitivity and high-rate thermal and thermodynamic measurements are needed. Because of their small size and rapid measurement speed, nanocalorimeters can be used for single measurements or with high throughput automation.

Continuous-flow enzyme assay on a microfluidic chip for monitoring glycerol secretion from cultured adipocytes

A dual-chip microfluidic platform that coupled perfusion of cultured adipocytes with on-line fluorescence-based enzyme assay was developed to monitor glycerol secretions in real time from cultured adipocytes. The perfusion cell chip, which could be reversibly sealed to allow reloading of cells and reuse of the chip, was shown by modeling to generate low shear stress on the cells under study. Effluent from the perfusion chip was pumped into an enzyme assay chip for monitoring of secretion from the cells. The on-line enzyme assay had a limit of detection (LOD) of 4 microM glycerol. The temporal resolution of the combined system for detecting changes in glycerol concentration was 90 s. The microfluidic device was used to continuously monitor glycerol secretion from murine 3T3-L1 adipocytes, grown and differentiated on glass coverslips, for at least 2 h. An average basal glycerol concentration of 28 microM was detected in the effluent. Pharmacological treatment with a beta-adrenergic agonist to stimulate lipolysis evoked a 3-fold increase in glycerol secretion followed by sustained release that was 40% higher than basal concentration. The ability to monitor changes in cellular secretion over time may provide insight into adipocyte metabolism and the dysregulation that occurs with obesity-related disorders.

High-sensitivity microfluidic calorimeters for biological and chemical applications

High-sensitivity microfluidic calorimeters raise the prospect of achieving high-throughput biochemical measurements with minimal sample consumption. However, it has been challenging to realize microchip-based calorimeters possessing both high sensitivity and precise sample-manipulation capabilities. Here, we report chip-based microfluidic calorimeters capable of characterizing the heat of reaction of 3.5-nL samples with 4.2-nW resolution. Our approach, based on a combination of hard- and soft-polymer microfluidics, provides both exceptional thermal response and the physical strength necessary to construct high-sensitivity calorimeters that can be scaled to automated, highly multiplexed array architectures. Polydimethylsiloxane microfluidic valves and pumps are interfaced to parylene channels and reaction chambers to automate the injection of analyte at 1 nL and below. We attained excellent thermal resolution via on-chip vacuum encapsulation, which provides unprecedented thermal isolation of the minute microfluidic reaction chambers. We demonstrate performance of these calorimeters by resolving measurements of the heat of reaction of urea hydrolysis and the enthalpy of mixing of water with methanol. The device structure can be adapted easily to enable a wide variety of other standard calorimeter operations; one example, a flow calorimeter, is described.

What heat is telling us about microbial conversions in nature and technology: from chip- to megacalorimetry

The exploitation of microorganisms in natural or technological systems calls for monitoring tools that reflect their metabolic activity in real time and, if necessary, are flexible enough for field application. The Gibbs energy dissipation of assimilated substrates or photons often in the form of heat is a general feature of life processes and thus, in principle, available to monitor and control microbial dynamics. Furthermore, the combination of measured heat fluxes with material fluxes allows the application of Hess' law to either prove expected growth stoichiometries and kinetics or identify and estimate unexpected side reactions. The combination of calorimetry with respirometry is theoretically suited for the quantification of the degree of coupling between catabolic and anabolic reactions. New calorimeter developments overcome the weaknesses of conventional devices, which hitherto limited the full exploitation of this powerful analytical tool. Calorimetric systems can be integrated easily into natural and technological systems of interest. They are potentially suited for high‐throughput measurements and are robust enough for field deployment. This review explains what information calorimetric analyses provide; it introduces newly emerging calorimetric techniques and it exemplifies the application of calorimetry in different fields of microbial research.

A microfabricated nanocalorimeter: design, characterization, and chemical calibration

A microfabricated titration calorimeter having nanowatt sensitivity is presented. The device is achieved by modifying a commercial, suspended-membrane, thin-film thermopile infrared sensor. Chemical reactions are studied by placing a 50.0 nL droplet of one reagent directly on the sensor and injecting nanoliter droplets of a second reagent through a micropipette by means of a pressure-driven droplet injector with 1% reliability in volume delivery. External thermal noise is minimized by a two-layer thermal shielding system. Evaporation is prevented by positioning the micropipette through a tiny hole in a cover glass, sealed by a drop of oil. The device is calibrated using two acid-base reactions: H2SO4 + HEPES buffer, and NaOH + HCl. The measured power sensitivity is 2.90(4) V/W, giving a detection limit of 22 nW. The 1/e time constant for a single injection is 1.1 s. The day-to-day power sensitivity is reproducible to approximately 2%. A computational model of the sensor reproduces the power sensitivity within 10% and the time constant within 20%. For a 50 nL sample and 0.8-1.5 nL titrant injection volumes, the heat uncertainty of 44 nJ corresponds to a 3sigma detection limit of 132 nJ, or the binding energy associated with 2.9 pM of IgG-protein A complex.

Microfabrication of screen-printed nanoliter vials with embedded surface-modified electrodes

A self-contained ion-selective sensing system within a nanoliter-volume vial has been developed by integrating screen printing, laser ablation, and molecular imprinting techniques. Screen printing and laser ablation are used in tandem to fabricate nanoliter-volume vials with carbon and Ag/AgCl ring electrodes embedded in the sidewalls. Using multisweep cyclic voltammetry, the surface of the carbon electrode can be modified with a polypyrrole film. By polymerizing pyrrole in the presence of nitrate, pores complementary to the nitrate anion in size, shape, and charge distribution are formed in the resulting film. Electrochemical cells modified with this nitrate-imprinted polypyrrole film show a near-Nernstian response to nitrate, and excellent reproducibility. The integration of molecular recognition and electrochemical response in the nanoliter vials is demonstrated by the detection of as little as 0.36 ng nitrate in nanoliter-volume samples. The integration of tailored molecular recognition within nanoliter vials via established fabrication and imprinting protocols should result in a number of nanosensor devices with applications in BioMEMS and micro total analysis systems.

Engineering challenges of BioNEMS: the integration of microfluidics, micro- and nanodevices, models and external control for systems biology

Systems biology, i.e. quantitative, postgenomic, postproteomic, dynamic, multiscale physiology, addresses in an integrative, quantitative manner the shockwave of genetic and proteomic information using computer models that may eventually have 10(6) dynamic variables with non-linear interactions. Historically, single biological measurements are made over minutes, suggesting the challenge of specifying 10(6) model parameters. Except for fluorescence and micro-electrode recordings, most cellular measurements have inadequate bandwidth to discern the time course of critical intracellular biochemical events. Micro-array expression profiles of thousands of genes cannot determine quantitative dynamic cellular signalling and metabolic variables. Major gaps must be bridged between the computational vision and experimental reality. The analysis of cellular signalling dynamics and control requires, first, micro- and nano-instruments that measure simultaneously multiple extracellular and intracellular variables with sufficient bandwidth; secondly, the ability to open existing internal control and signalling loops; thirdly, external BioMEMS micro-actuators that provide high bandwidth feedback and externally addressable intracellular nano-actuators; and, fourthly, real-time, closed-loop, single-cell control algorithms. The unravelling of the nested and coupled nature of cellular control loops requires simultaneous recording of multiple single-cell signatures. Externally controlled nano-actuators, needed to effect changes in the biochemical, mechanical and electrical environment both outside and inside the cell, will provide a major impetus for nanoscience.

Chip calorimetry for the monitoring of whole cell biotransformation

Efficient control of whole cell biotransformation requires quantitative real-time information about the thermodynamics and kinetics of growth and product formation. Heat production contains such information, but its technical application is restricted due to the high price of calorimetric devices, the difficulty of integrating them into existing bio-processes and the slow response times of established microcalorimeters. A new generation of chip or nanocalorimeters may overcome these weaknesses. We thus tested a highly sensitive chip calorimeter for its applicability in biotechnological monitoring. It was used to monitor aerobic growth of suspended and immobilized Escherichia coli DH5alpha DSM 6897 and anaerobic growth of suspended Halomonas halodenitrificans CCM 286(T). The chip data corresponded well with enthalpy balance calculations and measurements with a conventional calorimeter, indicating the applicability of the chip calorimeter for bio-process control.

Single-cell immunosensors for protein detection

A single-cell detector is described that combines the natural signal amplification of whole-cell biosensors with the flexibility and specificity of immunological recognition. An immune cell that expresses receptors for the constant region of immunoglobulin G (IgG) is loaded with a Ca(2+)-indicating dye and with antibodies directed against the protein of interest. Introduction of a multivalent protein antigen causes cross-linking of the receptors, which results in a detectable increase in the concentration of cytosolic Ca(2+). Some immune cell lines respond to stimulation with oscillations in their cytosolic Ca(2+) levels that complicate their use as detectors. The human monocytic cell line U-937, when treated with the cytokine interferon-gamma, produces a large, short-lived Ca(2+) signal in response to cross-linking of its high-affinity IgG receptors. U-937 was therefore chosen for development as an immunity-based detector. Human and rabbit antibodies are found to effectively stimulate the cell, causing a prompt and transient response. The cell is able to respond to repeated stimulation, though the response diminishes during rapid stimulation. Ovalbumin can be detected in micromolar concentrations. Possible fundamental constraints on the size of a detectable analyte are discussed.

Implementation of Multichannel Sensors for Remote Biomedical Measurements in a Microsystems Format

A novel microelectronic "pill" has been developed for in situ studies of the gastro-intestinal tract, combining microsensors and integrated circuits with system-level integration technology. The measurement parameters include real-time remote recording of temperature, pH, conductivity, and dissolved oxygen. The unit comprises an outer biocompatible capsule encasing four microsensors, a control chip, a discrete component radio transmitter, and two silver oxide cells (the latter providing an operating time of 40 h at the rated power consumption of 12.1 mW). The sensors were fabricated on two separate silicon chips located at the front end of the capsule. The robust nature of the pill makes it adaptable for use in a variety of environments related to biomedical and industrial applications.

A suspended membrane nanocalorimeter for ultralow volume bioanalysis

A nanocalorimetric suspended membrane sensor for pL volumes of aqueous media was fabricated by bulk silicon micromachining using anisotropic wet etching and photo and electron beam lithographic techniques. A high-temperature sensitivity of 125 microK and a rapid unfiltered time constant of 12 ms have been achieved by integrating a miniaturized reaction vessel of 0.7-nL volume on a 800-nm-thick and 300 x 300- microm2-large silicon nitride membrane, thermally insulated from the surrounding bulk silicon. The combination of a ten-junction gold and nickel thermoelectric sensor with an integrated ultralow noise preamplifier has enabled the resolution of 15-nW power in a single measurement, a result confirmed by electrical calibration. The combination of a high sensitivity and rapid response time is a consequence of miniaturization. The choice of gold and nickel as sensor material provided the maximum thermal sensitivity in the context of ease of fabrication and cost. The nanocalorimetric sensor has the potential for integration in an ultralow-volume high-density array format for the characterization of processes in which there is an exchange of heat.