344 Volume 58, Number 3, 2004 APPLIED SPECTROSCOPY0003-7028 / 04 / 5803-0344$2.00 / 0q 2004 Society for Applied Spectroscopy

spectroscopic techniques

Light Emission Diode Water Thermometer: A Low-Cost andNoninvasive Strategy for Monitoring Temperature inAqueous Solutions

SEBASTIAN A. THOMPSON, FRANCISCO J. ANDRADE, andFERNANDO A. INON*

Laboratorio de Analisis de Trazas, INQUIMAE-Departamento de Qu ‡mica Inorganica, Anal‡tica y Qu ‡mica F ‡sica–Facultad deCiencias Exactas y Naturales, Universidad de Buenos Aires. 3er Piso, Pabellon 2, Ciudad Universitaria. (C1428EHA) BuenosAires, Argentina

A spectroscopic device for monitoring the temperature of aqueoussolutions is presented. It uses a 950 nm light emission diode as lightsource and two photodiodes as detectors. Temperature is monitoredfollowing the thermally induced absorbance changes of the water–OH second overtone (;960 nm). A linear response between the lightabsorbed by an aqueous solution and its temperature is found inthe range from 15 to 95 8C. A prediction error of 0.1 8C and aprecision of 0.07 8C in temperature measurement can be achieved.Up to 0.1 M of electrolyte concentration can be present in the so-lution without signi� cantly affecting the temperature measurement.Different strategies, such as remote (noninvasive) or in situ (usinga � ber-optic probe) temperature measurement, are shown, andtheir relative advantages are discussed.

Index Headings: Spectroscopic temperature monitoring; NIR waterabsorption; Noninvasive temperature measurement.

INTRODUCTION

The most commonly used approaches for monitoringtemperature are based on the thermal expansion of liquids(e.g., mercury, alcohol, etc.) or the change of an electricalproperty (resistance, voltage, etc.) of a given device. Thislast strategy has shown some advantages over the former,although it also has some drawbacks; for example, whentemperature measurement should be performed in strongelectrical environments.1 In any case, all these approachesrely upon the heat transfer from the system under studyto the thermometer. This imposes serious spatial and timeconstraints to the measurement performed. As a matterof fact, because all of these ‘‘conventional’’ thermometerssense the temperature of the � uid close to them, theyalways act in an invasive way. Thus, the system understudy is disrupted by the presence of the thermometer andalso by the heat transfer required for the measurement.This problem increases when reducing the scale. Regard-ing temporal constraints, they need some amount of time,usually some seconds, to achieve thermal equilibrationwith the system. Thus, their proper use requires the as-sumption that the heat loss due to the measurement isnegligible and that variations to be detected are much

Received 26 August 2003; accepted 6 November 2003.* Author to whom correspondence should be sent. E-mail:

� non@qi.fcen.uba.ar.

larger than the thermometer response time. Therefore,when facing problems such as scale reduction, time-basedmeasurements, or spatial resolution of thermal gradients,these strategies can hardly be used. Some other problems,such as cross contamination when the temperature of sev-eral solutions must be measured, may also be present.

Thus, during the last decade, considerable interest hasbeen focused on the development of spectroscopic tem-perature sensors.2–6 Two different types of spectroscopicmeasurements of temperature, those related to bulk prop-erties and those concerning molecular attributes, can bementioned.4 In the � rst case, it is well known that theemission of the infrared radiation of a given object isnormally used as an indication of its temperature. How-ever, molecular-based measurements have attracted theattention of researchers during the last years, as they al-low the development of highly sensitive sensors and, insome cases, the resolution of spatial thermal gradients.2,5

The temperature-induced changes in the � uores-cence 2,3,5,6 or in the absorbance7 of different chemical sys-tems have been proposed for measuring the temperatureof different types of systems. With these approaches aprecision ranging between approximately 1 8C and 0.2 8Ccan be achieved. Some improvement of these values hasbeen reported 8 but at the expense of the practical appli-cability and versatility of the technique. In any case, allthese strategies require the addition of a compound to thesolution under study or its immobilization into a poly-meric matrix. This can be seen as one of the main draw-backs of these alternatives, due to the chemical restric-tions imposed on the system under study.

An alternative spectroscopic approach for monitoringtemperature in aqueous solutions that overcomes thisdrawback was reported during the last decade.9 The meth-od is based on the thermally induced spectroscopicchanges undergone by water in the near-infrared (NIR)region. Water shows two main NIR absorption bands,around 1400 and 970 nm, that correspond to the � rstovertone (FO) and second overtone (SO) of the OH–stretching, respectively.10 It is well known that when wa-ter is heated, these bands undergo a hypsochromic shiftand an increase in their intensity, which is ascribed to thedecrease in the degree of hydrogen bonding interac-

APPLIED SPECTROSCOPY 345

FIG. 1. (A) Instrumental setup of proposed optical thermometer device. (A) Operational ampli� er; (D1 and D2) photodiodes; (HM) half mirror;(S) light source; (L) lens; (T) thermostatable cell holder; (C) 1 cm optical pathlength cell. (B) Open-type � ber-optic cell.

tions.10,11 From an analytical point of view, this effect hasbeen mainly seen as a drawback that affects the perfor-mance of many NIR techniques, and several efforts havebeen devoted to overcoming or minimizing its in� uence(i.e., see Ref. 12). However, from a different perspective,Lin and Brown9 exploited this effect, developing a tem-perature sensor based on the thermally induced spectralchanges undergone by the FO. With an open-type cell� ber-optic probe and using principal component regres-sion (with 7 eigenvectors), a prediction error of 0.23 8Cin a range from 5 to 85 8C was reported. The method,however, required a complex instrumental setup and datatreatment because of the complexity of the spectralchanges involved and the working region of the spectra.

Applications of the second overtone have been lessstudied, perhaps due to its much lower intensity and itslocation in the short-wave NIR region, which is at theedge of the spectral range of commonly used spectro-photometers (either UV-VIS or NIR). Wulfert et al.,11

who analyzed the thermal � uctuations of this band usingmultivariate statistical analysis, concluded that a linearmodel would not explain the thermally induced peak � uc-tuations. Nonetheless, in a previous work13 we haveshown that if temperature prediction (instead of wholepeak variation explanation) is aimed for, thermally in-duced changes of the intensity of the SO can be linearlycorrelated with temperature, either by single or multiplelinear regression analysis. With this approach, tempera-ture of aqueous solutions was estimated by measuring theabsorbance at 958 nm using a conventional spectropho-tometer. This method allowed us to achieve a predictionerror of 0.7 8C in a range from 15 to 90 8C.

Several advantages of working with the second over-tone can be mentioned. From the spectroscopic point ofview, thermally induced spectral variations of the SOshow a lower degree of complexity, as peak shape andposition do not change as markedly as they do in the caseof the FO. This results in a considerable simpli� cation oftwo interdependent factors: the instrumentation and thedata analysis required.

In this work the instrumental advantages of monitoringtemperature using the NIR second overtone of water ab-sorption are exploited. These advantages arise from thesigni� cantly simpler instrumentation that can be used andthe optimal performance of the optical components in thisspectral range. Using a light emission diode (LED) and

a couple of photodiodes as detectors, a very sensitive,cheap, and noninvasive temperature sensor was devel-oped. Additionally, with the aid of a � ber-optic patchcord, a very sensitive temperature probe is also presented.In both cases, a linear response between temperature andlight absorption is observed in the range from 15 to 908C. The smallest temperature � uctuation that can be mea-sured with the proposed device was 0.07 8C and a pre-diction error of 0.1 8C was obtained.

EXPERIMENTALReagents and Solutions. Doubly deionized water

(DDW, 18 VM·cm21, Milli-Q Water system, Millipore,Bedford, MA) was used in all the experiments, unlessotherwise stated. Analytical grade reagents (NaCl, HCl,NaOH, Merck, Darmstadt, Germany) were used for ma-trix effect experiments. Stock solutions of 1 mol L21 wereprepared by weighing the appropriate amount of reagentand making up the volume to one liter. Appropriate di-lutions of these stock solutions were used for the rest ofthe experiments.

Apparatus and Software. Spectral information wasobtained using a multi-wavelength photodiode array(PDA) HP 8453 spectrophotometer (Hewlet Packard,Palo Alto, CA). For some experiments, the light outputof this spectrophotometer was blocked and a light emis-sion diode (LED) (LD271, OSRAM, Regensburg, Ger-many) with a maximum of emission at 950 nm was usedinstead. The LED was powered by a stabilized 5 V DCpower supply. A variable resistance was connected in se-ries with the LED in order to adjust the operating current.When no spectral discrimination was required, an instru-mental setup as shown in Fig. 1A was mounted using theLED as light source, a half mirror, and two photodiodes(BPX 81-3, Siemens, CA) as detectors. The signal wasacquired using a PC. In all the cases, LED light outputwas adjusted, checking that the detectors were not satu-rated.

A thermostatable cell holder provided with a cell-stir-ring module was used in all the experiments. A thermalwater circulator (MP Julabo Labortechnik, Germany) wasused for adjusting the cell-holder temperature. The tem-perature of a solution inside the cell was measured usinga thermistor connected to a conductimeter (Wissenschaf-tlich Technische Werkstatten LF 521). This system wascalibrated before the experiments using a platinum stan-

346 Volume 58, Number 3, 2004

dard resistance. The precision of the measurement of tem-perature using this system was 0.25 8C. All readings werecarried out through an acquisit ion board DAS-801(Keithley, Metrabyte). MatLab t was used for calcula-tions.

A � ber-optic patch cord (acrylic � ber optic commonlyused for illumination purposes and obtained from a localsupplier) 2.0 mm o.d. was properly � tted to the deviceshown in Fig. 1A. The ends of this � ber optic were usedfor making an ‘‘open’’ type9 � ber-optic sensor, as shownin Fig. 1B.

Procedure. A spectrophotometer cell furnished with amagnetic stirring bar was � lled with DDW and placedinto the cell holder. The thermistor was introduced intothe cell, checking that it does not interfere with the lightabsorption measurement. Temperature was varied be-tween 15 and 95 8C at intervals of approximately 5 8C.At each temperature, the spectrum of water was measuredonce the thermistor reading was stable by using the PDAspectrophotometer. This procedure was then repeated, re-placing the spectrophotometer light source with the LED.Finally, the same experiments were repeated using thesystem of Fig. 1A. In order to investigate the in� uenceof matrix composition on the obtained results, linear re-lationships between absorbance and temperature were in-vestigated in solutions with different total ionic concen-trations and different pH.

These last experiments were repeated using the � ber-optic probe described above. In these cases, the mea-surements were performed simply by immersion of theopen cell into the solution under study, whose tempera-ture was monitored using the thermistor.

RESULTS AND DISCUSSION

In our previous work (Fig. 1 in Ref. 13), we haveshown the short-wave NIR spectral variations observedwhen water is heated. As was mentioned above, peakattributes are the result of the balance between intra- andinter-molecular O–H interactions. Thus, these changes areusually ascribed to the change in the hydrogen bonding(HB) interactions between water molecules. In fact, whenan OH group forms a hydrogen bond, its stretching fre-quency is reduced and those stronger hydrogen bonds areassociated with greater reductions in frequency.14,15 Astemperature is increased, HBs are ‘‘broken’’, yielding anincrease in the intensity of the band of free –OH, whichis actually registered as a shift of the peak maximumtowards lower wavelengths.

However, there is no common agreement about the ac-tual number of component spectra that are involved ineach water band.10,16 As a matter of fact, the interpretationof these spectral changes relies upon the choice of oneof the many models of the water structure that have beenproposed.15 A mixture model depicts the structure of wa-ter at any instant as a mixture of a small number of dis-tinguishable species (i.e., local structures) of water mol-ecules. Other models depict the structure of water as anirregular network of molecules linked by distorted hy-drogen bonds.15 Experimental evidence supporting a mix-ture model has recently been reported, although there isno common agreement about the number and nature ofthe species involved. While Maeda et al.10 provide evi-dence of a stepwise hydrogen-bond breakage of waterspecies, Segtnan et al.16 suggest a two-state structural

model for water (weak or strong hydrogen bonded watermolecules).

Beyond the proposed models, chemometric and spec-troscopic tools have shown the complexity of the studyof the thermally induced spectral changes when the aimis to explain the whole peak variation.10,11,16,17 Neverthe-less, if the aim of the work is the prediction of temper-ature, practical correlations can be found using only somewavelengths. In fact, we have shown13 that if measure-ments are performed at a single wavelength (958 nm), alinear dependence between SO absorbance and tempera-ture can be established. Similar results were reported forthe FO.9,10 Under a mixture model assumption, these re-sults could be explained considering that when selectingan appropriate wavelength only one single water species(whose concentration increases as temperature rises) isinvolved in the measurement.

From a different point of view, these results suggestthat an adequate monochromatic light source and a de-tector could replace the spectrophotometer used, thusleading to a considerable simpli� cation of the instrumen-tation required. From the different types of light sourcesavailable, LEDs have received considerable attention dur-ing the last years because they are stable and low-costdevices.

The main drawback of using LEDs in analytical spec-troscopy is related to their spectral distribution, that is,their maximum peak position and spectral bandwidth.Fortunately, for the purpose of this work, the � rst draw-back can be easily overcome because there is a commer-cially available LED with a maximum of emission at 950nm, thus overlapping with the water SO absorption. Theproblem that still remains is whether the concept of‘‘monochromatic light’’ can be applied to them. In fact,monochromaticity is not an absolute, but a relative valuethat arises from comparing the source bandwidth and thebandwidth of the spectrum under study.

When comparing LED emission spectra and water SOabsorption pro� les (at different temperatures), it showsthat the LED bandwidth at half of the maximum is almost50 nm, which is comparable to that of the water secondovertone (;80 nm). Thus, to be fair, according to thetheory18 one should not expect Beer’s law to hold in thissystem.

Nonetheless, when the experimental setup shown inFig. 1A is used, a linear dependence between temperatureand absorbance is found. The dependence obtained canbe described by the equation:

T 5 0.0036·A 1 8.06

where T is the temperature and A is the detector responsein mV, which is directly related to the absorbance. Thelinear dependence holds from 15 to 90 8C, and the pre-diction error for temperature is 0.1 8C. The signal-to-noise ratio obtained allows a discrimination of 0.07 8C.

Once again, it should be stressed that there is no apriori reason for expecting the linear behavior obtained.The only sensible explanation should be related to somekind of compensation arising from the complexity of thesystem. As a matter of fact, from the chemical point ofview, it was already mentioned that the thermally inducedchanges on water spectra can be correlated with simul-taneous changes in several species, in a way that has notyet been clearly stated. Furthermore, the LED spectral

APPLIED SPECTROSCOPY 347

FIG. 2. Spectral pro� les using the LED as light source and DAD as detector. (A) Transmittance pro� les. 100% of transmittance was set using aW lamp as source. The arrows over the water pro� les indicate increasing order of temperature of the solution (from 20 to 85 8C). (B) Absorbancepro� les of the same solutions as A, but using the LED emission pro� le (‘‘No cell’’) as 100% of transmittance. Arrows in the water pro� le indicateincreasing temperature of the solution.

distribution overlaps with the region of these changes,making it dif� cult to determine the exact nature andamount of the species involved.

From a phenomenological perspective, if the area un-der the absorption pro� les presented in our previous workis calculated (setting the LED bandwidth as limits of in-tegration, i.e., between 900 and 1000 nm) a linear de-pendence with temperature is obtained. The slope, inter-cept, and correlation coef� cient are equal to 20.46 nm8C21, 2187 nm, and 0.9909, respectively.

Some insight into the spectral behavior of the systemcan be gained by resolving the water absorption pro� lewhen the LED is used as the source. For doing this, a

diode array detector was used. First, experiments withoutusing water were performed in order to characterize theLED emission pro� le. It was found that, as expected,when the operating currents are increased from 2.5 to 15mA, peak emission intensity grows. However, when nor-malizing these pro� les (i.e., dividing all the values by thevalue at the peak maximum), all the peaks overlap, exceptwhen saturation of the detector occurs. This is importantbecause, depending on the type of measurement to beperformed (i.e., in situ or noninvasive), and in order toachieve the best signal-to-noise ratio, it allows the re-searcher to adjust the power supply of the proposed de-vice without affecting the spectral attributes.

348 Volume 58, Number 3, 2004

FIG. 3. Typical temperature jump using the � ber-optic sensor.

Secondly, the experiments where water temperaturewas changed were repeated under these new conditions.Figure 2A shows the intensity pro� les for the experi-mental conditions previously described . Absorbancespectra can be calculated by taking the ratio of each spec-trum against the LED emission spectra (Fig. 2B). Onlythe region between 900 and 990 nm shows a suitablesignal-to-noise ratio, as the intensity of the LED outsidethese boundaries is negligible. Absorption peak pro� lesare not only sharply different from those obtained usingthe standard DAD spectrophotometer, but also muchmore noisy. Nevertheless, if the area under the absor-bance pro� le is calculated (integration window 900–990nm), it yields a linear relationship with temperature (Abs5 0.0275 Temp 1 12.6; R 2 5 0.957). The relatively lowcorrelation coef� cient value is ascribed to the low signal-to-noise ratio, which is a drawback arising from the useof the DAD.

Regarding the possibility of using the proposed device(Fig. 1A) with a probe, the open cell depicted in Fig. 1Bwas immersed into the water bath and the absorbance wasmonitored. Although the noise level is slightly higherwhen compared with the glass cell, it can be reduced bya proper increase of the operating current. As the tops ofthe � ber optic are mounted in screw-type connectors (Fig.1B), different optical paths can be tested. Pathlength val-ues up to 1 cm yield results similar to those describedabove, providing that the current is adjusted to obtain thesame signal-to-noise level (Fig. 3). For pathlength valuesabove 1 cm, no good results are obtained (more noiseand poor sensitivity), but this may be caused by the great-er degree of light dispersed. In fact, in the absence of alight-collimating device, a higher optical path produceslight dispersion that cannot be easily compensated withthe increase of the LED operating current.

It is well known that many solutes, particularly dis-solved ions, may distort the absorption pro� le of wa-ter.1,19,20 Therefore, 0.1 M solutions of commonly usedelectrolytes (NaOH, HCl, NaCl) were tested. No signi� -cantly different results from those obtained with pure wa-ter were found (data not shown), which is in agreementwith the results published in our previous work.13

CONCLUSIONA highly sensitive and versatile device for measuring

temperature of aqueous solution in a very precise and

simple way is presented. It can be applied to a plethoraof chemical systems without the problems arising fromthe addition of special reporter molecules. Furthermore,the use of the SO instead of the FO presents some prac-tical advantages, such as the possibility of using longeroptical paths and the minimization of the effects of ther-mal expansion of the sensor cell. The instrumental setupis simple and very accessible, as it does not require eitherpeak scanning or complex data treatment. Furthermore,the analytical performance is comparable to, or even bet-ter than, many other techniques. It should be stressed thatthe components required, such as the � ber-optic patchcord, present excellent performance in this spectral range.The same is true for silicon photodiodes, whose responseis optimum at the working wavelengths.

Some improvements regarding this methodology, suchas correction of non-speci� c absorbance, are at presentbeing performed. Considering the nature of the measuredproperty, some work is being conducted in order to per-form spatially and time-resolved thermal gradients and toscale down the system.

A deeper understanding of the linear dependence ofthe integrated absorbance on solution temperature foundrequires a deeper knowledge of the water structure, whichis still under revision.16 It is not clear if the observedchanges are only due to variations of the concentrationof different water species or if there exists some depen-dence of the water OH integrated absorption coef� cienton temperature.17

ACKNOWLEDGMENTS

The authors want to thank Dr. Mabel Tudino for her support andadvice, and Mr. Sergio Gomez for his collaboration.

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