In Situ Thermal Imaging and Absolute Temperature Monitoring by

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In Situ thermal imaging and absolute temperature monitoring by luminescent diphenylalanine nanotubes Zhixing Gan, Xinglong Wu, Jinlei Zhang, Xiaobin Zhu, and Paul K Chu Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/bm400562c • Publication Date (Web): 16 May 2013 Downloaded from http://pubs.acs.org on May 22, 2013

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In Situ thermal imaging and absolute temperature monitoring by luminescent diphenylalanine nanotubes Zhixing Gan†, Xinglong Wu*,†, Jinlei Zhang,† Xiaobin Zhu†, and Paul K. Chu‡ † Key Laboratory of Modern Acoustics, MOE, Institute of Acoustics and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing, 210093, P. R. China ‡ Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China KEYWORDS: Diphenylalanine nanotubes, Luminescence properties, Thermometer ABSTRACT: The temperature sensing capability of diphenylalanine nanotubes is investigated. The materials can detect local rapid temperature changes and measure the absolute temperature in situ with a precision of 1 oC by monitoring the temperature-dependent PL intensity and lifetime, respectively. The PL lifetime is independent of ion concentrations in the medium as well as pH in the physiological range. This biocompatible thermal sensing platform has immense potential in the in situ mapping of micro-environmental temperature fluctuations in biological systems for disease diagnosis and therapeutics.

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Introduction Precise monitoring of intracellular temperature is crucial to the study of cell activities.1-6 It has been reported that pathological cells have a higher temperature than normal cells due to the enhanced metabolic activity.2,7,8 In fact, the temperature of living cells is affected by events such as cell division, gene expression, enzymatic reactions, and metabolism,2 and so accurate knowledge of the local cellular temperature is important to disease diagnosis2,7,8 as well as therapeutic treatment.5,7 The photothermal method has been developed for cancer therapy5,9,10 but hyperthermal treatments may damage healthy organs and tissues without precise temperature monitoring.5 Hence, a technique to map the local temperature in a micro-biological environment such as microcircuits11,12 and microfluids with the submicrometer scale is necessary.13-16 The development of a luminescent temperature sensing platform which is noninvasive, accurate, and able to resist strong electromagnetic fields with micrometer and nanometer spatial precision is of both scientific and technical importance.7,8,17 The mechanism is based on the temperature-dependent photoluminescence (PL) properties like intensity,2,5,18,19 peak shape,20-26 polarization3, spectral shift, 27,28 and lifetime1,13,15,16,29-31 of materials such as organic dyes,13,14,23 quantum dots,18,19,21,24,28 and rare-earth-doped materials.17,22,30-32 In spite of recent progress, these materials suffer from drawbacks and limitations,7,8,16 such as the inability to determine the absolute temperature, toxicity in biological systems, poor solubility, photobleaching, and so on.2,5,7,18,19

Furthermore, luminescent measurements may suffer from fluctuations due to

variations in the local sensor concentration thereby rendering the measurement of the absolute temperature inaccurate. Ratiometric luminescent temperature sensors based on Cd-related binary or ternary semiconductors (e.g. CdSe, CdS and CdSeTe) can measure the absolute temperature.21,24,26 A core–shell structure is normally preferred to isolate and protect the toxic


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Cd-based core, but release of toxic heavy metal ions Cd stemming from the degradation of QDs is inevitable7,33 thus undermining the biocompatibility. Much effort has been made to explore peptide-based nanostructures in biomedicine and nanotechnology since they are biocompatible, modifiable, and relatively simple in structure. 34,35 Diphenylalanine (FF), which constitutes the core recognition motif of Alzheimer’s β-amyloid peptide, is a suitable bottom-up building block34,36,37 and nanostructures composed of FF have high stability against proteolytic attacks and can work under harsh chemical conditions such as extreme pH and organic solvents with different polarities.38,39 FF nanostructures have been applied to drug delivery systems,35 supercapacitor electrodes,40 probing of water molecules,36,37 and second harmonic generation.41

In this work, the temperature-dependent PL and time-

resolved PL (TRPL) spectra acquired from FF nanostructures are studied in details to assess their role as a precise temperature sensor.

Both the PL intensity and lifetime show a strong

dependence on the temperature and the intensity variation provides a convenient means to detect rapid temperature changes in situ while the lifetime conveys the absolute temperature. Materials and Methods Details of the sample preparation has been described elsewhere.42,43 The FF microtubes were produced in a homemade cylindrical chamber containing a fixed volume of water and a clean slide was placed in the chamber. The fresh FF solution was prepared by dissolving FF powders in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) to a concentration of 40 mg mL-1 by sonication (150 W, 40 kHz). Afterwards, 30 µL of the FF solution was dripped on the slide for 60 min. The FF microtubes were then formed through a self assembly process. The formation mechanism of microtubes and more structural characterization (X-ray diffraction, Raman scattering and


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differential scanning calorimeter) can be found in our previous work.36,37,42,43 The FF nanotubes were obtained by cleaving the microtubes. The microtubes were immersed in ethanol for 24 h, mechanically agitated (Vortex Shaker, Keer), and then let settled for several minutes. Since the surface of microtube is hydrophobic,44 the large-size microtubes quickly sink down, while the FF nanotubes suspend in ethanol due to their small sizes. The precipitates (big microtubes) were removed and the supernatant containing the FF nanotubes were acquired. Finally, the powders of the FF nanotubes were obtained after evaporating the ethanol. The powder of FF nanotubes (4 mg) were redispersed in deionized water (2 mL) and encapsulated in a quartz cuvette. In the temperature-dependent PL measurements, the cuvette with the specimen was placed in a holder while water was pumped around the holder from an external circulating temperature bath. The PL and fluorescence lifetime were measured on an Edinburgh FLS-920 PL spectrometer. The excitation and monitoring emission wavelengths were 256 and 282 nm, respectively and the instrumental response of the system was measured using ludox elastic scattering. In the pH-dependent TRPL measurements, the FF nanotubes were dispersed in PBS with different pH. The pH value was adjusted by the 0.1 M KH2PO4 and K2HPO4 solution and the K+ concentration was 150 mM. The SEM images were acquired on the Hitachi S-3400N II scanning electron microscope (SEM). Results and discussion The FF microtubes are synthesized using the protocols reported previously.36,37,42,43 As shown in Fig. 1a, the microtubes have lengths of up to several hundred micrometers and diameters of about 20 µm. The large size limits its application in micro-environmental temperature sensing and in order to improve the spatial resolution and solubility, the microtubes are cleaved to form


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nanotubes by immersing them in ethanol under vigorous stirring. Afterwards, the suspension can be separated into two parts, big microtubes with a step-like morphology (precipitates, Fig. 1b) and small nanotubes (supernatant, Fig. 1c) by stilling for several minutes. The lengths of the nanotubes depend on physical agitation and some short nanotubes can be obtained to have lengths less than 300 nm (the inset of Fig. 1c). Our previous investigation has indicated that the PL peak positions and lifetimes mainly depend on the content of water molecules in nanochannels.36 The size effect of the nanotubes is negligible. The dimensions of the nanotubes are below the thickness of microchips and size of microfluidic devices (hundreds of micrometers)13-16 rendering them feasible in in situ thermal imaging. More importantly, common cancer cells have dimensions of several to dozens of microns and these small fragments can serve as intracellular temperature sensors due to cellular uptake of nanostructures.45,46 Fig. 2 depicts the temperature-dependent PL and TRPL spectra of the FF nanotubes. As shown in Fig. 2a, the PL intensity decreases dramatically as the suspension temperature is increased from 5 to 65 oC. By setting the intensity of the strongest PL peaks at the lowest temperature as 1, the intensity at a higher temperature exhibits a proportional trend. The circles in Fig. 2c show the relationship between the PL intensity and temperature. In the physiological temperature range between 25 and 45 oC, the PL intensity decays by about 39.2%, corresponding to a decreases 1.96% per oC. Assuming that it is difficult to distinguish a 1% variation in the spectrum, the FF nanotubes are capable of detecting temperature changes with a sensitivity of about 0.5 oC, which is adequate in monitoring intracellular temperature variations in biological processes which require a temperature resolution of about 0.5 °C1-4,6 and also competitive compared to other reported luminescent thermal sensors.2,7,8,14,19,24,30,32

For example, the typical temperature

changes in opto-fluidic devices caused by laser radiation are on the order of several degrees8 and


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the cancer cell temperature increases by more than ten degrees Celsius under laser radiation during photothermal therapy.5,6,9 Fig. 2b shows that with increasing temperature, the PL decay becomes faster and the PL lifetime also diminishes monotonically. The lifetime is obtained by fitting the TRPL curves with a single exponential decay function. It should be mentioned that the current room-temperature PL lifetime is shorter than that reported previously (17.2 ns).43 Previously, the time-resolved PL decay curve was obtained from the powder-like FF microtubes, while currently all the time-resolved PL decay curves were measured from the FF nanotubes dispersed in de-ionized water. The recombination of electron-hole pairs is easily affected by the polarization field42 and the polarity of the solvent may be responsible for the lifetime difference in different environments. The relationship between the PL lifetime and temperature is plotted in Fig. 2c (squares) and it can also be fitted approximately by an exponential decay function as follows:

τ f (T ) = 15.17 exp(−T / 43.94) − 1.84 .

(1)

According to Reference,8 the ability of a luminescent system is usually gauged by the normalized lifetime thermal coefficient, ατ(T) defined as ατ(T) = dτnor(T) / dT, where τnor(T) = τf (T) / τf (25 oC) is the normalized PL lifetime at temperature T relative to room temperature. As a result, ατ(T) of the FF nanotubes can be calculated as:

ατ (T ) = dτ nor (T ) / dT = 0.0515 exp(−T / 43.94) .

(2)

At 0 oC, ατ(T) is as large as 0.0515 per oC. When the temperature is increased to 35 oC, ατ(T) is about 0.0232 per oC which is still a competitive value.7,8 Although organic dyes embedded in polymers and layer double hydroxides exhibit higher relative sensitivity, they function in a narrower temperature range (~10 oC) than these luminescent nanomaterials7 and also cannot


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cover the entire physiological temperature range (several dozen degrees) in photothermal therapy.5,9,10 In addition to monitoring the local temperature changes by measuring the PL intensity, the absolute temperature can be determined by the PL lifetime and so this sensing platform serves two important purposes simultaneously. The relationships of the PL intensity and PL lifetime versus temperature are nearly the same (Fig. 2c) because the intrinsic mechanisms of these two effects are similar.

At a higher

temperature, inelastic scattering of excitons by optical or acoustic phonons reduces the exciton lifetime, resulting in the corresponding reduction in the radiative state population and consequent emission.47 The temperature dependence of the experimental lifetime may be approximated by the Mott-Seitz model:7,17,18,29,31,32

τ −1 = τ r−1 + k exp(− ∆E / k BT ) ,

(3)

where τr is the radiative lifetime, k is the pre-exponential factor, ∆E is the energy gap between the emitting level (EL) and the higher excited-state level (HESL), and kB is Boltzmann’s constant. As shown in Fig. 3, a high temperature activates the phonon-assisted process. The thermally assisted energy transfer process accelerates the non-radiative events, increases the net deexcitation probability of the emitting level, and eventually reduces the PL lifetime and intensity.7 The experimentally obtained lifetime values are not affected by the FF concentrations. As shown in Fig. 2d, when the concentration of FF nanotubes is varied from ~2 to 1 and 0.5 mgmL-1, the three PL decay curves are almost identical. It has been reported that the blinking properties of fluorescent protein are sensitive to pH and buffer composition22 and therefore, a specific calibration curve must be obtained for each buffer.2,22 However, our experiments indicate (Fig. 4a) that the fluorescence decay process inherent to FF nanotubes is almost independent of the


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environmental pH in the physiological range because the 282 nm emission peak stems from band-to-band recombination in the FF nanotubes which have a band gap depending on the amount of water molecules in the nanochannels.36 Since the FF nanotubes are in an aqueous environment, no addition or loss of water takes place in the nanochannels and the PL properties do not vary. This is a unique advantage of this sensing platform because the local pH in living cells is frequently affected by neighboring structures and can vary by as much as ±1.2 The total ion concentration in cytoplasm is 150 mM (K+ is the most abundant)2 and the phosphate buffered saline (PBS) used here for the lifetime measurements also contains about 150 mM K+. However, the lifetime derived from the PBS is almost the same as that determined from FF nanotubes dispersed in deionized water showing unambiguously that the lifetime at the same temperature is constant regardless of ionic concentrations. However, it should be noted that the PL decay curves obtained from FF nanotubes in PBS with a pH of less than 5.5 show slightly longer PL lifetime (Fig. 4b), possibly due to protonation of oxygen-containing functional groups.48 The FF nanotube thermal sensing platform thus works best in the physiological pH range between 6.0 and 8.0. The FF nanotubes are used to monitor random temperature variations controlled by an external circulating bath. The temperature at six points of A-F is set randomly and Fig. 4c displays the PL decay curves at these six points. By fitting the lifetime τ and inserting τ to the PL lifetime versus temperature function, the temperature is determined and shown in Fig. 4d. Compared to the temperature detected by a K-type thermocouple, the discrepancy is less than 1 oC.

The

temperature fluctuation caused by the thermocouple can be ignored since its size is quite small and so this discrepancy can be attributed to two factors. The first one is that it takes a finite time to acquire the PL decay curve and during this period of time, the temperature is not strictly


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constant. In comparison, the temperature measured by the thermocouple is instantaneous. The second factor is the sum of the normal errors of both a thermocouple (an error of at least 0.5 oC) and our sensing materials.

All in all, the temperature accuracy of the FF nanotube-based

platform should be better than 1 oC which is also comparable to that reported previously.49

Conclusion An in situ thermal imaging and absolute temperature monitoring platform comprising bioinspired FF nanotubes is described. The absolute temperature can be determined from the PL lifetime and rapid thermal fluctuations can be monitored by the PL intensity simultaneously. The temperature sensing capability is independent of ionic concentrations and environmental pH in the physiological range. Hence, accurate mapping of local temperature in micro-environments such as microcircuits and microfluids is possible by these biocompatible FF nanotubes which offer excellent spatial resolution, sensitivity, and accuracy.


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Figure 1. Fabrication process of the FF nanotubes: (a) As-prepared microtubes; (b) Cleaved microtubes with a step morphology; (c) Nanotubes with a size of about 5 µm detached from the microtubes. The corresponding SEM images are also shown and the inset in the SEM image of nanotubes (c) demonstrates the existence of some fragments shorter than 300 nm.


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Figure 2. (a) Temperature-dependent PL and (b) TRPL spectra of the FF nanotubes. The IRF in (b) is the instrumental response function. (c) Plots of the average PL intensity (blue circle) and PL lifetime (black square) versus temperature. The black line is the fitted lifetime as a function of temperature. The fitted values and corresponding standard errors are listed in the table in the inset.

The error bars represent standard deviations from the average values from three

independent measurements. (d) PL decay curves of the FF nanotubes in aqueous solutions at different concentrations.


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Figure 3. Schematic drawing of the qualitative mechanism explaining the temperature dependent PL lifetime.


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Figure 4. TRPL decay curves of the FF nanotubes in PBS with different pH values of (a) 6.5-7.5 and (b) 5.5-8.0. (c) PL decay curves at six random temperature points of A-F. (d) Plots of the PL lifetime values (blue triangles) fitted from the six decay curves in (c). The temperature calculated from the PL lifetime and detected by a thermocouple is also plotted as black squares and red circles, respectively.


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AUTHOR INFORMATION

Corresponding Author Corresponding authors – Electronic mails: [email protected] (X.L.W.)

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was jointly supported by Grants (Nos. 2011CB922102 and 2013CB932901) from the Basic Research Programs of China and PAPD. Partial support was also from the Hong Kong Research Grants Council (RGC) General Research Funds (GRF) No. CityU 112510 and 112212. REFERENCES 1.

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In Situ thermal imaging and absolute temperature monitoring by luminescent diphenylalanine nanotubes Zhixing Gan, Xinglong Wu*, Jinlei Zhang, Xiaobin Zhu, and Paul K. Chu

PL lifetime Fitting result PL intensity

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Table of Contents Graphic

PL lifetime (ns)

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Biomacromolecules

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Temperature ( C)


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In Situ Thermal Imaging and Absolute Temperature Monitoring by

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