Electron Spin Resonance Dipstick

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Electron Spin Resonance Dipstick Oleg Zgadzai, Ygal Twig, Helen Wolfson, Rizwan Ahmad, Periannan Kuppusamy, and Aharon Blank Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00917 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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Analytical Chemistry

Electron Spin Resonance Dipstick

Oleg Zgadzai,1 Ygal Twig,1 Helen Wolfson,1 Rizwan Ahmad,2 Periannan Kuppusamy,3 and Aharon Blank1*

1

Schulich Faculty of Chemistry, Technion – Israel Institute of Technology, Haifa 3200008, Israel

2

Department of Biomedical Engineering, Ohio State University, Columbus, OH 43210, USA 3

Departments of Radiology and Medicine, Geisel School of Medicine, Dartmouth College, Lebanon, NH 03756, USA

*Correspondence

Aharon Blank PhD Schulich Faculty of Chemistry Technion – Israel Institute of Technology Haifa 3200008, Israel [email protected]

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Abstract

Electron spin resonance (ESR) is a powerful analytical technique used for the detection, quantification, and characterization of paramagnetic species ranging from stable organic free radicals and defects in crystals to gaseous oxygen. Traditionally, ESR requires the use of complex instrumentation, including a large magnet and a microwave resonator in which the sample is placed. Here, we present an alternative to the existing approach by inverting the typical measurement topology, namely placing the ESR magnet and resonator inside the sample rather than the other way around. This new development relies on a novel self-contained ESR sensor with a diameter of just 2 mm and length of 3.6 mm, which includes both a small permanent magnet assembly and a tiny (~1 mm in size) resonator for spin excitation and detection at a frequency of ~ 2.6 GHz. The spin sensitivity of the sensor has been measured to be ~ 1011 spins/√Hz and its concentration sensitivity is ~0.1 mM, using reference samples with a measured volume of just ~10 nL. Our new approach can be applied for monitoring the partial pressure of oxygen in vitro and in vivo through its paramagnetic interaction with another stable radical, as well as for simple on-line quantitative inspection of free radicals generated in reaction vessels and electrochemical cells via chemical processes.


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Introduction Electron spin resonance (ESR) is a well-established analytical method commonly used to identify and quantify paramagnetic species ranging from stable organic free radicals to dopants, defects and impurities in crystals, and unique endohedral systems.1 ESR is traditionally pursued by taking an aliquot of the sample to be measured, placing it in a container tube, and measuring it in a large magnetic field with high homogeneity. This poses severe limitations on the applicability of the method for a variety of chemical, life science, and medical applications. For example, for in situ monitoring of free radicals generated by chemical reactions, it is necessary to construct highly cumbersome experimental setups for flowing the reactants through the magnet, or to conduct the reactions inside the ESR cavity.2-4 Due to the complexity and the limitations of this approach, many resort to suboptimal solutions for in-situ detection of radicals, either through their indirect effects on fluorescence signals5 or through the use of non-linear highfrequency response of resonance coils.6 Similar and even more severe problems arise in the context of in vivo measurements, where extremely large magnets and complex and costly systems are required in order to measure paramagnetic species inside the body of animals or humans.7 Accordingly, it would be highly beneficial if ESR could be developed to a point where it can be employed in a much simplified manner in in situ and in vivo applications that conform with the abovementioned examples and in many other timely applications. In this work, we take a significant step toward the realization of this vision by developing a unique “ESR Dipstick” system. Our approach inverts the common paradigm of placing the examined sample or subject inside the magnet and resonator and suggests a new topology where the magnet and the ESR resonator are placed inside the 3 ACS Paragon Plus Environment


sample. This allows the possibility of performing in-situ and in-vivo ESR measurements of a variety of paramagnetic samples simply by inserting the ESR dipstick directly inside the sample of interest.

Experimental Section

Figure 1: (a) Conceptual design and operation of the ESR Dipstick sensor, used either for in-vitro or in -vivo analysis. (b) Exploded view of the sensor, showing its individual parts. (c) Drawing of the fully assembled sensor. (d) Picture of the head of the assembled sensor, shown with a millimeter-scale ruler.


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Design concept: The general concept, mechanical assembly, and working principles of the new ESR sensor are described in Fig. 1. The sensor can be used either for in vitro monitoring of processes occurring in a test tube or a reactor, or to monitor ESR signals inside animals or humans. Accordingly, the signal picked up by the sensor can originate from endogenous paramagnetic species generated in a chemical reaction or process, or from exogenous spin probes that come integral with the sensor. In the case of in vivo monitoring, it is practically impossible to get a measurable signal from endogenous radicals (due to their low concentration and short lifetime); in such cases, the sensor can be used primarily for monitoring partial pressure of oxygen (pO2), pH, and thiols through their effect on specific exogenous stable spin probes that can be located inside the sensor.8,9 The diameter of the current sensor is 2 mm, and the length of its head is 3.6 mm. These dimensions were chosen to allow it to be used in in vivo applications through a 12-gauge histology needle. The fully integrated ESR sensor includes a pair of permanent magnets and a tuned miniature loop-gap resonator operating at ~2.5 GHz. The sensor measures signals originating primarily at the center of the resonator (Fig. 1c), to where solvent can freely flow from the examined sample, or alternatively, it can fixate a stable free radical used for environmental pO2 or pH monitoring (as mentioned above, mainly in the context of in-vivo measurements). While most ESR systems operate in continuous wave (CW) mode, namely, they measure the ESR signal by fixing the frequency and scanning the static field, the miniature sensor is intended for pulsed operation. In pulsed-mode ESR, the magnetic field is fixed and short microwave pulses are applied to cover the extent of the measured ESR spectrum. This eliminates the requirement for magnetic field scanning and thus greatly simplifies the design of the probe. Furthermore, in pulsed operation, the



requirements from static magnetic field homogeneity are much less stringent than in CW, since echo sequences can be used to obtain the signal even in a grossly inhomogeneous field.10 The magnet of the miniature ESR sensor: The magnet was designed using Maxwell finite element software by Ansoft. The design is shown in Fig. 2. It is based on two miniature permanent magnets made of temperature-compensated samarium-cobalt (material SmCo 2:17TC-7 from Electron Energy Corporation, USA, Br=0.55T). Each cylinder is 1.6 mm in diameter and 1 mm in height, magnetized along the z-axis. In addition, two shim steel (type 1010) plates are added (1.6 mm in diameter, 0.4 mm high). The aim of the design was to reach a field of ~95 mT, which is as homogenous as possible. This field, corresponding to a frequency of ~2.66 GHz, is compatible with our current pulsed ESR spectrometer’s spectral range (see below), and also advantageous in terms of having relatively low dielectric losses in skin tissue environments.11,12 Due to the small dimensions of the sensor, we could not map out its magnetic field using, for example, an ultra-small Hall probe, and had to rely on the calculated results and the actual ESR signal obtained in our experiments. These experiments revealed that the calculated field often deviates by as much as ±100 Gauss from the calculated data, most probably due to production inaccuracies and material variations.


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Figure 2: (a) Drawing of the magnet assembly, showing the direction of the magnetic field around the location of the examined sample at the center of the magnet structure (marked by an orange dot). (b) The calculated static field along the x-axis (blue) and z-axis (red), about the center of the magnet structure.



Figure 3: (a) Drawing of the resonator structure (dimensions are in mm). (b) The calculated magnetic (H) and electric (E) microwave fields at resonance frequency inside the resonator (cut in the XZ plane). The magnetic field at the center of the resonator is calculated to be 5.5 G/√W. (c) Picture of the bare resonator structure. (d) Measured reflection coefficients (S11) for the resonator placed in different environments (air – blue line, distilled water – red line, tap water – green line, and acetone – magenta line). In addition, calculated (CST Microwave Studio) values are shown for the air environment (black stars). The resonator: The design of the resonator faces several challenges. It must be rather small in size (~2 mm) to fit between the set of miniature permanent magnets, and at the same time operate at a relatively low frequency of ~2.6 GHz, where the wavelength (of about 120 mm) is much larger than the resonator’s size. Moreover, the resonator should be minimally affected when operating in aqueous media and also enable the free flow of liquids in/out of it for sample measurements. Based on these requirements, we have come 8 ACS Paragon Plus Environment

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up with the loop-gap resonator13 design shown in Fig. 3, which evidently conforms to all of them. As a starting point we used the known empirical formulas for the inductance and capacitance of loop-gap resonator structure:13

L= C = ε rε 0

µ0π r 2 , z + 0.9r

(1a)

( w + t )( z + t ) , t

(1b)

where r is the inner radius of the loop (assuming it is round), z is the height of the loop, w is the width of the loop, and t is the gap width (see Fig. 3a). If we consider, for example, a resonator with a small loop having r~0.5 mm and z~1.5 mm that would fit our intended 2-mm structure, the inductance would be around L~0.5 nH. Thus, based on the known LC circuit resonance frequency expression of f =

1 2π LC

, this means that the capacitance

should be C~8 pF to obtain the required resonance frequency of f~2.6 GHz. A quick look at eq. (1b) reveals that this required gap capacitance is much larger than that which we can achieve for the gap in our tiny resonator with reasonable machining tools (i.e., not smaller than 100 µm), which is only ~0.7 pF, even when using dielectric filling of εr=10. This notion led to the use of a small lumped capacitor, which can also be easily modified or replaced if needed to control the exact resonance frequency and match it to the static field of the sensor. Following this initial reasoning, we made a more accurate design of the resonator structure with the aid of finite element software (CST Microwave Studio). The resonator is made integral with the entire brass casing of the sensor, which holds the magnets and thus simplifies the assembly process. In addition to its dimensions



and resonance frequency, the resonator is characterized by a modest quality factor (Q) of ~35-20 (the lower value is obtained when moving from air to an aqueous environment), as measured (Fig. 3d), and a relatively large magnetic field conversion factor of ~20 G/√W, as calculated (Fig. 3b). Another important parameter for characterizing the resonator and the system’s sensitivity is its effective volume, Vc,14,15 which is equal to the volume of a small hypothetical sample Vv (for example [1 µm]3, usually located at the point where the resonator’s microwave magnetic field is maximal), divided by the filling factor4 of this small sample. In our case, Vc was calculated to be ~0.12 microliter. ESR Spectrometer: The microwave system that drives the sensor is based on our homebuilt pulsed ESR system.

A detailed description of the system is provided

elsewhere.16,17 For experiments conducted within the present framework, we extended the spectrometer’s operating frequency range down to the 2-4 GHz. The upgraded system also includes the capability to produce arbitrary-shaped pulses using an arbitrary waveform generator (AWG) card with 0.4-ns time resolution (WavePond model DAx22000-8M Chase Scientific Co., USA).

Results and Discussion The sensor’s capabilities in terms of its spin sensitivity and concentration sensitivity were characterized both theoretically and experimentally. In order to calculate its expected performance, we must first analyze the relatively complicated profiles of its static field, B0, and its microwave (MW) magnetic field, B1. The relatively small size of the magnets leads to inhomogeneous distributions of B0, and the relatively small resonator 10 ACS Paragon Plus Environment

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results in an inhomogeneous B1. These fields, as depicted in Fig. 4a-b, vary considerably in magnitude over the 3D region of interest (ROI). In addition, they also vary in their orientation. Therefore, in order to verify the applicability of the design, the net ESR signal should be calculated through the superposition of signals from individual spatial locations (voxels), which may vary in amplitude and phase.18 Here, we set the frequency of the excitation pulses to be equal to the average value of γB0 in the ROI (γ is the electron gyromagnetic ratio). The excitation bandwidth is controlled by adjusting the pulse’s width and/or shape, while the flip angle is controlled by adjusting its magnitude. An example of the predicted ESR signal amplitude in various parts inside the sensor, for a Hahn echo sequence employing 30-ns-long rectangular pulses, is provided in Fig. 4c. Overall, it is calculated (and evident from Fig. 4c) that a volume of ~0.01 micro-liter at the center of the sensor is accessible for ESR signal acquisition. For such short excitation pulses, the magnetic field in the measured volume has inhomogeneity of ~10 Gauss (see also Fig. 2b). Longer excitation pulses with a lower B1 field would result in picking up signals from smaller volumes having a higher field homogeneity, at the expense of losing sensitivity.



Figure 4: (a) The static magnetic field profile at the center of the sensor. (b) The microwave magnetic field in the same region, as plotted in (a). (c) The calculated relative ESR signal from various parts of the sensor for the same region at the center, as plotted in (a) and (b). 12 ACS Paragon Plus Environment

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Based on these calculations, we can now derive the expected spin sensitivity of the sensor using the expression (eq. A1) listed in the Appendix, for two types of typical samples.

The first sample is a lithium octa-n-butoxy-naphthalocyanine (LiNc-BuO)

radical19 used for measuring pO2. Using eq. (A1) with the sensor properties listed above, for a sample having T2* of ~100 ns (due to the field inhomogeneity of the sensor) and T1 of 10 µs (both of which are typical of a LiNc-BuO sample in anoxic conditions), at room temperature, gives a predicted spin sensitivity of ~1010 spins/√Hz. Alternatively, for an aqueous sample (with degraded Q-factor) of trityl free radicals20, with similar T1 and T2*, spin sensitivity would be ~3×1010 spins/√Hz. Using these spin sensitivity numbers and the predicted volume from which a signal is obtained (of ~0.01 microlitter – see above), we can estimate the predicted signal-tonoise-ratio (SNR) for cases in which the sensor is used for oxygen sensing with LiNc-BuO crystals, and for measuring trityl radicals in a solution. In the former case, since there are ~3×108 spins/µm3 of LiNc-BuO,17 we can expect to have ~1015 spins in our sensitive volume, resulting in an expected SNR of ~105 for 1 second of acquisition time, when all conditions are optimized. Alternatively, in the case of trityl, for a solution of 1 mM there should be ~6×1012 spins in the measurement volume, meaning that the expected SNR for this case would be ~300, when all other conditions are optimized. To verify the performance of the sensor we carried out measurements with test samples. The first sample was made of solid LiNc-BuO crystals that filled the resonator’s volume (Fig. 5a). The sample was subjected to an argon flow to eliminate the presence of oxygen. We used a simple Hahn echo sequence with two pulses of equal length (30 ns) and an inter-pulse separation, τ, of 200 ns. The results of these measurements are shown 13 ACS Paragon Plus Environment


in Fig. 5c. The signal is ~4.5 volts and the RMS noise is ~1.3 mV, giving a SNR of ~3300. However, to fully optimize the sensor’s sensitivity, the entire echo signal can be integrated, thus improving the results by a factor of ~4.5, leading to a total SNR of ~14,800 for 1 second of averaging in our measurement. This is less than the predicated SNR mentioned above. However, given the uncertainties of the excited volume size and the finite T2 of the solid particulates that degrades the signal (our calculation assumed optimal condition and does not relate to T2 decay), the results are very encouraging and clearly show the capability of the current sensor to easily address a variety of in-vitro and in-vivo pO2 sensing applications19 by means of its unique configuration. In that respect, we have carried out additional tests to show that the pO2 can indeed be quantified using measurements of the T2 by Hahn echo sequence with varying inter-pulse distance, τ, and comparing the measured value to the 1/T2 vs. pO2 calibration curve of the radical (Fig. 5e-f).


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Figure 5: (a) Picture of the sensor with the LiNc-BuO sample placed inside it (black particulates). (b) Picture of the sensor inside the trityl solution. (c) Measured ESR echo signal for the LiNc-BuO sample. (d) Measured ESR echo signal for the 1-mM trityl solution (red line) and the noise level of the measurement (magenta line). The scale is the same as the one used for the LiNc-BuO plot. (e) The Hahn echo sequence, repeated with different inter-pulse separation, τ, which makes it possible to quantify the T2 of the radical in the sensor. (f) The measured value of T2 can be used to quantify pO2 with a linear 1/T2 vs pO2 calibration curve, measured with calibrated premixed N2 and O2 gas compositions.



Further to the measurement of the solid particulates, we also measured a 1-mM solution of trityl free radical in water. We employed an echo sequence similar to the one used for the LiNc-BuO particulates (first and second pulse length, 60 ns, τ = 100 ns). The measurement setup and the results of these measurements are shown in Fig. 5b and in the inset of Fig. 5c. Clearly, due to the much lower spin concentration of this sample, the SNR is greatly degraded to ~27 (after Fourier transformation of the time domain data and integration over the relevant spectral window, compared to the RMS noise value with the same processing). This is also lower than the expected optimal values, but given the abovementioned issues concerning excitation uncertainties and finite T2, it’s not too far off from our calculations. It can be concluded that the experimentally measured spin sensitivity for the solid LiNc-BuO sample is ~7×1010 spins/√Hz, that for the trityl sample it is 2×1011 spins/√Hz, and that the concentration sensitivity is ~37 µM/√Hz. Commercial ESR spectrometers operating at X-band have a much better absolute spin sensitivity (~109-1010 spins/√Hz) and concentration sensitivity (less than 1 µM), since they operate at higher fields, with optimized high Q resonators and much larger sample volumes. Commercial systems aimed for in-vivo work operating at lower fields, similar to the one we use in our sensor, have much lower sensitivity (~1013 spins/√Hz).21

Of course, this sensitivity reduction is the

price that must be paid for the convenience of the measurement protocol offered here with its unique sample-sensor topology. Comparison to previous state-of-the-art: Table 1 provides information on the two most relevant works (in our opinion) concerning compact self-contained magnetic resonance sensors. In terms of NMR, ref

22

covers a great deal of configurations and


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devices. Based on this seminal work, it is clear that while many designs with micro-coils in the micron scale exist for NMR, they are not incorporated into miniature magnet designs and mostly adopt the classical NMR approach of having large homogenous magnets and placing the sample in or near the magnet and sensing coil. In fact, the smallest magnet + coil design published in the field comes from our own group (the first entry in Table 1). This NMR sensor indeed resembles the dimensions of the current ESR sensor, but its sample probing volume is much larger, and it is intended for inspection of samples near its magnet and sensing volume (rather than going into the sample in our case). Moreover, the fact that it is an NMR sensor with a resonance frequency 3 orders of magnitude smaller than that of our current ESR proposal makes it a much different design, with different challenges and much different applications. In terms of other sensors of relevance carrying out ESR measurements, the most relevant work that was recently published is described in the second entry of Table 1.23 This sensor is far off from the work presented here in terms of magnet size as well as geometry of measurement (sensor is placed near the sample and not inserted into the sample), mode of operation (CW and not pulsed), and intended applications.



Ref. # 24

23

Magnet size 1.8 φ×10 mm

Resonator/ Probed coil size volume 1.5 mm ~400 nl

15.9 φ×7.9 mm ~100 µm

~1 pL

Sensitivity and remarks

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Sensor design

NMR sensor, ~2-8 MHz, sensitivity of ~1018 protons/√Hz

CW ESR sensor, absolute sensitivity of ~109 spins/√Hz. Concentration sensitivity of ~20mM/√Hz. Field scan and modulation coils need to be added to enable CW detection and spectrum scan.

Table 1: Configurations of compact self-contained NMR and ESR sensors of relevance to the present work


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Conclusions – Prospects and Limitations We have presented a new approach to conduct ESR experiments in a manner that is completely opposite to the currently established designs. With the new approach, the sensor, representing the ESR system, is inserted into the sample rather than the sample being placed inside the ESR system. The configuration described here can be of immediate use for sensitive pO2 sensing applications in the fields of chemistry and biology/medicine, based on the high SNR obtained with the LiNc-BuO particulates. This high SNR can enable even smaller designs in the future for pre-clinical and even clinical applications. Such medical applications would follow the path laid out by recent work on bioimplantable ESR resonators for oxygen sensing (but without having the integral miniature magnet presented in our work), involving extensive in-vivo studies.7,25 These studies have already addressed important aspects such as biocompatibility (through encapsulation by a medical grade oxygen penetrable silicone elastomer); transferring the microwave energy in/out of the resonator by special transmission lines not prone to breaks (made of molybdenum alloy); and coupling energy through the skin via inductive coupling schemes. The measurement of radicals in a solution is more challenging, although for that purpose a slightly larger sensor can be developed with higher magnetic field homogeneity and a larger sensing volume, thus greatly increasing the concentration sensitivity of the measurement. In the future, the sensor described here can be incorporated into miniature ESR-systems-on-a-chip26,27 currently being developed, potentially resulting in a simple, portable, and affordable complete solution for pO2 sensing and free radicals measurement. It should be noted that due to the probe’s low frequency and high field inhomogeneity, the sensor cannot be expected to achieve high spectroscopic resolution to enable the 19 ACS Paragon Plus Environment


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identification of different or unknown radicals. However, it could be used to quantify radical concentration, based on calibrated test samples common in analytical techniques, assuming the radical’s identity is known a priori.

Acknowledgements This work was partially supported by NIH grants R21 EB016189 and R21 EB022247.

Appendix: Spin sensitivity The spin sensitivity of the sensor can be obtained using the expression:28 ≈ Sensitivity spins Hz

8 Vc kbT (1/ π T2* )

µ Bω0 2µ0

ω0 Qu

T1 BF

(A1) ,

where Vc was described above, kb is the Boltzmann constant, T is the temperature in which the experiment is carried out (assumed to be the same for the spins and the resonator), and

(1/ π T2* ) = ∆f is the bandwidth of signal acquisition, chosen to match the spin-spin relaxation time (including static inhomogeneities), T2*. In cases where T2 is long enough to enable a multi-echo acquisition sequence (such as CPMG), the sensitivity is improved accordingly and T2* can be replaced by T2.

The variable ω0 is the Larmor angular

frequency, Qu is the loaded quality factor of the resonator, T1 is the spin-lattice relaxation


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BF =

time, BF is the Boltzmann population factor

1+ e 1− e

−

ω0 k BT

−

ω0 k BT

, h is Planck’s constant, µ0 is

the free space permeability, and µ B is the Bohr magneton.

References (1) Misra, S. K.; Wiley-VCH: Berlin, 2011. (2) Borrowman, C. K.; Zhou, S. M.; Burrow, T. E.; Abbatt, J. P. D. Phys. Chem. Chem. Phys. 2016, 18, 205-212. (3) Gilbert, B. C.; King, D. M.; Thomas, C. B. Journal of the Chemical Society, Perkin Transactions 2 1981, 1186-1199. (4) Poole, C. P. Electron spin resonance : a comprehensive treatise on experimental techniques, 2nd ed.; Wiley: New York, 1983, p xxvii, 780 p. (5) Miller, K. E.; Burch, E. L.; Lewis, F. D.; Torkelson, J. M. J Polym Sci Pol Phys 1994, 32, 2625-2635. (6) Hong, H.; Krause, H. J.; Song, K.; Choi, C. J. Appl. Phys. Lett. 2012, 101, 054105 (7) Swartz, H. M.; Williams, B. B.; Zaki, B. I.; Hartford, A. C.; Jarvis, L. A.; Chen, E. Y.; Comi, R. J.; Ernstoff, M. S.; Hou, H. G.; Khan, N.; Swarts, S. G.; Flood, A. B.; Kuppusamy, P. Acad. Radiol. 2014, 21, 197-206. (8) Ahmad, R.; Kuppusamy, P. Chem. Rev. 2010, 110, 3212-3236. (9) Khramtsov, V. V.; Grigor'ev, I. A.; Foster, M. A.; Lurie, D. J. Antioxid. Redox Signal. 2004, 6, 667676. (10) Blumich, B.; Perlo, J.; Casanova, F. Prog. Nucl. Magn. Reson. Spectrosc. 2008, 52, 197-269. (11) Baker-Jarvis, J. R.; Kim, S.; Leschallinger, L.; Johnson, J.; Givot, B., Characterization of TissueEquivalent Materials for High-Frequency Applications (200 MHz to 20 GHz); NIST2010. (12) Sasaki, K.; Mizuno, M.; Wake, K.; Watanabe, S. In 2015 40th International Conference on Infrared, Millimeter, and Terahertz waves (IRMMW-THz), 2015, pp 1-2. (13) Rinard, G. A.; Eaton, G. R. In Biomedical EPR, Part B: Methodology, Instrumentation, and Dynamics, Eaton, S. R.; Eaton, G. R.; Berliner, L. J., Eds.; Springer US: Boston, MA, 2005, pp 19-52. (14) Mims, W. B. In Electron Paramagnetic Resonance, S., G., Ed.; Plenum Press: New York,, 1972, pp 263-351. (15) Blank, A.; Dunnam, C. R.; Borbat, P. P.; Freed, J. H. J. Magn. Reson. 2003, 165, 116-127. (16) Blank, A.; Suhovoy, E.; Halevy, R.; Shtirberg, L.; Harneit, W. Phys. Chem. Chem. Phys. 2009, 11, 6689-6699. (17) Shtirberg, L.; Twig, Y.; Dikarov, E.; Halevy, R.; Levit, M.; Blank, A. Rev. Sci. Instrum. 2011, 82, 043708. (18) Wolfson, H.; Ahmad, R.; Twig, Y.; Williams, B.; Blank, A. Health Phys. 2015, 108, 326-335. (19) Pandian, R. P.; Raju, N. P.; Gallucci, J. C.; Woodward, P. M.; Epstein, A. J.; Kuppusamy, P. Chem. Mater. 2010, 22, 6254-6262. (20) Ardenkjaer-Larsen, J. H.; Laursen, I.; Leunbach, I.; Ehnholm, G.; Wistrand, L. G.; Petersson, J. S.; Golman, K. J. Magn. Reson. 1998, 133, 1-12. 21 ACS Paragon Plus Environment


(21) Cazes, J.; Ewing, G. W. Ewing's analytical instrumentation handbook, 3rd ed.; Marcel Dekker: New York, 2005, p xxiv, 1037 p. (22) Zalesskiy, S. S.; Danieli, E.; Bluemich, B.; Ananikov, V. P. Chem. Rev. 2014, 114, 5641-5694. (23) Campbell, J. P.; Ryan, J. T.; Shrestha, P. R.; Liu, Z. L.; Vaz, C.; Kim, J. H.; Georgiou, V.; Cheung, K. P. Anal. Chem. 2015, 87, 4910-4916. (24) Blank, A.; Alexandrowicz, G.; Muchnik, L.; Tidhar, G.; Schneiderman, J.; Virmani, R.; Golan, E. Magn. Reson. Med. 2005, 54, 105-112. (25) Caston, R. M.; Schreiber, W.; Hou, H. G.; Williams, B. B.; Chen, E. Y.; Schaner, P. E.; Jarvis, L. A.; Flood, A. B.; Petryakov, S. V.; Kmiec, M. M.; Kuppusamy, P.; Swartz, H. M. Cell Biochem. Biophys. 2017, 75, 275-283. (26) Handwerker, J.; Schlecker, B.; Wachter, U.; Radermacher, P.; Ortmanns, M.; Anders, J. In IEEE International Solid- State Circuits Conference - (ISSCC), 2016 (27) Yang, X.; Babakhani, A. IEEE Journal of Solid-State Circuits 2016, 51, 2408-2419. (28) Twig, Y.; Suhovoy, E.; Blank, A. Rev. Sci. Instrum. 2010, 81, 104703.


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For TOC only



Figure 1: (a) Conceptual design and operation of the ESR Dipstick sensor, used either for in-vitro or in -vivo analysis. (b) Exploded view of the sensor, showing its individual parts. (c) Drawing of the fully assembled sensor. (d) Picture of the head of the assembled sensor, shown with a millimeter-scale ruler. 190x107mm (300 x 300 DPI)

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Figure 2: (a) Drawing of the magnet assembly, showing the direction of the magnetic field around the location of the examined sample at the center of the magnet structure (marked by an orange dot). (b) The calculated static field along the x-axis (blue) and z-axis (red), about the center of the magnet structure. 275x397mm (300 x 300 DPI)



Figure 3: (a) Drawing of the resonator structure (dimensions are in mm). (b) The calculated magnetic (H) and electric (E) microwave fields at resonance frequency inside the resonator (cut in the XZ plane). The magnetic field at the center of the resonator is calculated to be 5.5 G/√W. (c) Picture of the bare resonator structure. (d) Measured reflection coefficients (S11) for the resonator placed in different environments (air – blue line, distilled water – red line, tap water – green line, and acetone – magenta line). In addition, calculated (CST Microwave Studio) values are shown for the air environment (black stars). 190x107mm (300 x 300 DPI)


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Figure 4: (a) The static magnetic field profile at the center of the sensor. (b) The microwave magnetic field in the same region, as plotted in (a). (c) The calculated relative ESR signal from various parts of the sensor for the same region at the center, as plotted in (a) and (b). 275x397mm (300 x 300 DPI)



particulates). (b) Picture of the sensor inside the trityl solution. (c) Measured ESR echo signal for the LiNcBuO sample. (d) Measured ESR echo signal for the 1-mM trityl solution (red line) and the noise level of the measurement (magenta line). The scale is the same as the one used for the LiNc-BuO plot. (e) The Hahn echo sequence, repeated with different inter-pulse separation, τ, which makes it possible to quantify the T2 of the radical in the sensor. (f) The measured value of T2 can be used to quantify pO2 with a linear 1/T2 vs pO2 calibration curve, measured with calibrated premixed N2 and O2 gas compositions. 275x397mm (300 x 300 DPI)


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Figs in Table 1 338x190mm (96 x 96 DPI)



TOC fig 338x190mm (96 x 96 DPI)


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Electron Spin Resonance Dipstick

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