**3.3. Polymer nanofiber laser**

#### *3.3.1. Dye Doped Polymer MNF Knot Resonator Laser*

Recently, many kinds of microsized polymer fiber and dye doped polymer fiber were also successfully demonstrated. Q. Song et al. reported the lasing action in a dye doped PNF knot ring resonator [74]. The dye doped polymer nanofiber was fabricated by general fiber drawing technique. Then it was bent to a knot ring resonator and fixed onto glass tubes (the inset of Fig. 18a), which can increase the stability of the knot resonator and fix its diameter [74]. Laser emission was observed. The emission spectrum of knot resonator is shown in Fig. 18b. Here the pump intensity is 4.2 mJ/cm2. Periodic peaks can be observed in the laser spectrum. This group of peaks is considered to come from the whispering galley mode resonance inside the ring resonator. The linewidth of laser peaks is only 0.07 nm and the mode spacing in Fig. 18 is about 0.206 nm.

**Figure 18.** Laser spectrum of dye doped polymer fiber [74].

input blue light.

**3.3. Polymer nanofiber laser** 

mode spacing in Fig. 18 is about 0.206 nm.

*3.3.1. Dye Doped Polymer MNF Knot Resonator Laser* 

**Figure 17.** Optical microscope images of nanofibers with a rugby-ball-like QDs-decorated cross structure and a cascaded rugby-ball-like QDs-decorated structure [73]. (a) Crossed structurewith a guided 473 nm blue light in nanofiber 2 and excited 630 nm red light at the cross junction. The inset schematically representsthe cross structures. (b) Cascaded rugby-ball-like QDs-decorated structure. The lengths of the QDs B1 to B7are 2.3, 2.2, 2.3, 2.2, 1.8, 2.0, and 2.0 μm, respectively. The corresponding maximum widths are 1.6, 1.5, 1.6, 1.6, 1.0, 1.1, and 1.1 μm. (c) Rugby-ball-like QDs-decorated fiber excited by 473 nm blue light at *P*ex = 1.2 μW. The blue arrow shows the direction of propagation of the

Recently, many kinds of microsized polymer fiber and dye doped polymer fiber were also successfully demonstrated. Q. Song et al. reported the lasing action in a dye doped PNF knot ring resonator [74]. The dye doped polymer nanofiber was fabricated by general fiber drawing technique. Then it was bent to a knot ring resonator and fixed onto glass tubes (the inset of Fig. 18a), which can increase the stability of the knot resonator and fix its diameter [74]. Laser emission was observed. The emission spectrum of knot resonator is shown in Fig. 18b. Here the pump intensity is 4.2 mJ/cm2. Periodic peaks can be observed in the laser spectrum. This group of peaks is considered to come from the whispering galley mode resonance inside the ring resonator. The linewidth of laser peaks is only 0.07 nm and the

#### *3.3.2. Random lasering in a singleorganic Nanofiber*

One-dimensional light random lasing in individual p-sexiphenyl nanofibers (p-6P) is investigated by F. Quochi et al [36]. The laser action happens in single p-6P nanofibers grown on (001)-oriented muscovite mica. Isolated nanofibers are shown to yield lowthreshold random laser emission in the deep blue. Random lasing from isolated nanofibers starts at pump fluences on the order of 10 μJ/cm2 per pulse. Lasing nanofibers are demonstrated. Figure 19a shows a lasing micrograph taken in imaging mode slightly above threshold. It displays both lasing from vertically aligned nanofibers and spontaneous emission from a set of neighboring nanofibers oriented approximately at 60° with respect to the vertical axis of the detection system. The latter faintly appear in the lower part of the graph. Scattering of the lasing emission into out-of-plane directions does not take place homogeneously along the nanofibers' axis; conversely, scattering is highly spotted, indicating that wave-guiding is interleaved with light scattering and outcoupling at special sites along the fibers. Emission spectra relating to the ~100 μm long nanofiber placed at the center of the imaging field of view in Fig. 19a are reported in Fig. 19b. They refer to the emission spatially integrated over the whole nanofiber length. Below threshold, spontaneous emission exhibits a broad vibronic progression with 0–1 and 0–2 emission bands peaked near 425 and 450 nm, respectively.

#### *3.3.3. Optically pumped lasing in single conjugated polymer nanowires*

Conjugated polymers have chemically tuneable opto-electronic properties and are easily processed, making them attractive materials for photonics applications [75,76]. Conjugated polymer lasers, in a variety of resonator geometries such as microcavity, micro-ring, distributed feedback and photonic bandgap structures, have been fabricated using a range of coating and imprinting techniques [77-80]. D. O'Carroll et al. reported the first observation of optically pumped lasing in single conjugated polymer nanowires [81]. The waveguide and resonator properties of each wire are characterized in the far optical field at room temperature. The end faces of the nanowire are optically flat and the nanowire acts as a cylindrical optical cavity, exhibiting axial Fabry–Pérot mode structure in the emission spectrum. Above a threshold incident pump energy, the emission spectrum collapses to a single, sharp peak with an instrument-limited line width that is characteristic of singlemode excitonic laser action. Fluorene-based conjugated polymers are attractive.

**Figure 19.** (a) Gray-scale optical emission intensity image of lasing and luminescent *p*-6P nanofibers excited at a pump fluence (Φ) of 15 *μ*J/cm2 per pulse. The gray-level scale is logarithmic. The *y*  coordinate refers to the position along the vertical direction, which is parallel tothe input slit of the detection system. (b) Emission intensity spectra of the nanofiber positioned at *x* ≈ 30 *μ*m in panel a and extending vertically from *y* ≈ 50 *μ*m to *y* ≈ 150 *μ*m for different values of the pump fluence. Note that the threshold fluence is lower than 12 μJ/cm2 per pulse [36].

Fluorene-based conjugated polymers are attractive photonic materials because they exhibit high photoluminescence quantum efficiencies, large stimulated emission crosssections and chemically tuneable emission wavelengths [82]. Poly(9,9-dioctylfluorene) (PFO) is a prototypical main-chain liquid-crystalline homopolymer that emits in the blue and exhibits polymorphic behaviour, with striking implications for its photophysical properties [83]. Isolated PFO nanowires were uniformly excited by the 355-nm output of a 0.7 ns, 1.25 kHz pulsed Nd/YVO laser and spatially resolved photoluminescence spectra were acquired from the bodies and tips (Fig. 20a). To confirm that the wires operated as axial Fabry–Pérot microcavities, the mode spacing at 460 nm was plotted versus the inverse nanowire length for 14 wires, and was shown to exhibit a linear dependence (Fig. 20b). The number of Fabry– Pérot modes per guided mode that could propagate in a nanowire microcavity was estimated from Δλspont/Δλm, where Δλspont is the full-width-at-half-maximum, FWHM, of the spontaneous emission and is ~19 nm.

Single isolated PFO nanowire microcavities were then uniformly excited (355 nm, 0.7 ns, 1.25 kHz) and tip emission spectra were collected as a function of pump energy (Fig. 21a). At lower pump energies, tip spectra exhibited nanowire microcavity emission, with pronounced Fabry–Pérot modes apparent at the 0–1 peak. Above an energy threshold of ~100 nJ (2.8 mJ cm–2), a single spectrally narrowed emission peak developed by preferential gain in a single Fabry–Pérot mode and the onset of lasing. A slight blue shift in peak position with increasing pump energy suggested that stimulated emission occurred on timescales comparable to or faster than exciton energy migration. Concerning the dependence of tip emission intensity on pumpenergy, below the energy threshold, emission increased linearly with excitation energy (Fig. 21b). Above threshold, a kink in emission output was followed by a super-linear increase due to optical gain. Also, the emission peak width narrowed from 19.6 nm to 1.4 nm (instrument-limited) at threshold, indicating the high quality factor of the nanowire cavity.

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a cylindrical optical cavity, exhibiting axial Fabry–Pérot mode structure in the emission spectrum. Above a threshold incident pump energy, the emission spectrum collapses to a single, sharp peak with an instrument-limited line width that is characteristic of single-

**Figure 19.** (a) Gray-scale optical emission intensity image of lasing and luminescent *p*-6P nanofibers excited at a pump fluence (Φ) of 15 *μ*J/cm2 per pulse. The gray-level scale is logarithmic. The *y*  coordinate refers to the position along the vertical direction, which is parallel tothe input slit of the detection system. (b) Emission intensity spectra of the nanofiber positioned at *x* ≈ 30 *μ*m in panel a and extending vertically from *y* ≈ 50 *μ*m to *y* ≈ 150 *μ*m for different values of the pump fluence. Note that the

Fluorene-based conjugated polymers are attractive photonic materials because they exhibit high photoluminescence quantum efficiencies, large stimulated emission crosssections and chemically tuneable emission wavelengths [82]. Poly(9,9-dioctylfluorene) (PFO) is a prototypical main-chain liquid-crystalline homopolymer that emits in the blue and exhibits polymorphic behaviour, with striking implications for its photophysical properties [83]. Isolated PFO nanowires were uniformly excited by the 355-nm output of a 0.7 ns, 1.25 kHz pulsed Nd/YVO laser and spatially resolved photoluminescence spectra were acquired from the bodies and tips (Fig. 20a). To confirm that the wires operated as axial Fabry–Pérot microcavities, the mode spacing at 460 nm was plotted versus the inverse nanowire length for 14 wires, and was shown to exhibit a linear dependence (Fig. 20b). The number of Fabry– Pérot modes per guided mode that could propagate in a nanowire microcavity was estimated from Δλspont/Δλm, where Δλspont is the full-width-at-half-maximum, FWHM, of the

Single isolated PFO nanowire microcavities were then uniformly excited (355 nm, 0.7 ns, 1.25 kHz) and tip emission spectra were collected as a function of pump energy (Fig. 21a).

threshold fluence is lower than 12 μJ/cm2 per pulse [36].

spontaneous emission and is ~19 nm.

mode excitonic laser action. Fluorene-based conjugated polymers are attractive.

**Figure 20.** Microcavity effects in single PFO nanowires [81]. a, Emission spectra collected from the tip (blue) and body (grey) of an isolated PFO nanowire under uniform pulsed excitation (1.4 nJ). Inset: emission image of an excited wire (1nJ) with spatially filtered emission images of the tip and body locations from which spectral data were acquired; mauve lines indicate the area over which each spectrum was integrated. Scale bars, 2 μm. b, Plot of mode spacing measured at 460 nm versus inverse nanowire length for 14 different nanowires. Black squares, experimental data points; green triangle, extrapolation to infinite length; dashed blue line, fit of Fabry–Pérot equation to data for *λ* =460 nm and [n –*λ*(d*n*/d*λ*)] = 5.4. Inset: schematic depiction of a nanowire with well-defined end facets acting as a Fabry–Pérot microcavity.

**Figure 21.** Optically pumped single PFO nanowire laser [81]. a, Emission spectra collected from the tip of an isolated PFO nanowire under uniform excitation, as a function of increasing pump energy at room temperature. Inset: emission image of the wire (1.3 nJ) along with spatially filtered emission images of the tip and body locations from which spectral data were acquired. Scale bars, 2 μm. b, Plot of the tip emission peak intensity (blue squares) and FWHM (red triangles) versus pump energy for the wire shown in a. The intensity of emission from the nanowire body (green squares) is low and almost linear with pump energy. Solid symbols correspond to experimental data points and lines are guides to the eye. Inset: above threshold (230 nJ) emission spectra acquired from the wire tip (blue) and body (green).

#### **3.4. Polymer nanofiber photodetector**

Semiconducting polymers are attractive materials due to their chemically tunable optical and electronic properties, as well as their facility for solution processing [84,85]. Garret A. O'Brien et al. have demonstrated that solution-assisted template wetting may be successfully exploited for high-yield controlled synthesis of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(bithiophene)] (F8T2) nanofibes and employed single nanowire devices as ultraminiature photodetectors. [35] Discrete nanowires with average lengths of 15 μm and mean diameters of 200 nm has been fabricated. As expected, structural data point to a low degree of crystallinity within the wires. Individual nanowires can be electrically interfaced using either bottom- or top-contact geometries. Top-contacted single-nanowire devices with interelectrode gaps of approximately 5 m were fabricated on glass substrates using shadow masking and gold evaporation, see Fig. 22a. The blue curve in Figure 22b shows the measured dark current (Idark) for a typical device. The data show quasilinear characteristics at low bias, similar to measured data for bottom-contacted nanowires, with some asymmetry at higher bias. The red curve in Fig. 22b shows the measured current (Iillum) under continuous 405 nm illumination. A marked increase in the measured current is observed across the entire bias range. The green curve in Fig. 22b shows the measured current under manually chopped 405 nm illumination, where the illumination was switched on or off at 10 V intervals during the bias sweep, corresponding to a duty cycle of 50 %. The present photoconductivity measurements of F8T2 nanowire devices, which yield single-nanowire responsivities of approximately 0.4 mAW–1 and external quantum efficiencies of approximately 0.1% under monochromatic illumination; these values are comparable with data reported for single-inorganic-nanowire devices. The results demonstrate the promise of these novel nanostructures as ultraminiature photodetectors with the potential for integration into future hybrid nanophotonic devices and systems.

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**Figure 21.** Optically pumped single PFO nanowire laser [81]. a, Emission spectra collected from the tip of an isolated PFO nanowire under uniform excitation, as a function of increasing pump energy at room temperature. Inset: emission image of the wire (1.3 nJ) along with spatially filtered emission images of the tip and body locations from which spectral data were acquired. Scale bars, 2 μm. b, Plot of the tip emission peak intensity (blue squares) and FWHM (red triangles) versus pump energy for the wire shown in a. The intensity of emission from the nanowire body (green squares) is low and almost linear with pump energy. Solid symbols correspond to experimental data points and lines are guides to the eye. Inset: above threshold (230 nJ) emission spectra acquired from the wire tip (blue) and body (green).

Semiconducting polymers are attractive materials due to their chemically tunable optical and electronic properties, as well as their facility for solution processing [84,85]. Garret A. O'Brien et al. have demonstrated that solution-assisted template wetting may be successfully exploited for high-yield controlled synthesis of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(bithiophene)] (F8T2) nanofibes and employed single nanowire devices as ultraminiature photodetectors. [35] Discrete nanowires with average lengths of 15 μm and mean diameters of 200 nm has been fabricated. As expected, structural data point to a low degree of crystallinity within the wires. Individual nanowires can be electrically interfaced using either bottom- or top-contact geometries. Top-contacted single-nanowire devices with interelectrode gaps of approximately 5 m were fabricated on glass substrates using shadow masking and gold evaporation, see

**3.4. Polymer nanofiber photodetector** 

**Figure 22.** a) Schematic of a top-contact nanowire device. b) I–V characteristicsof a top-contacted F8T2 nanowire acquired in the dark (blue line) and under 405 nm illumination (red line). Reversible switching in I–V data measured under manually chopped 405 nm illumination is alsoshown (green line). Inset: optical microscopy image of a typical top-contacted F8T2 nanowire device. Scale bar: 10 m.[35]

### **3.5. Nanopatterning of single light-emitting polymer nanofibres**

Polymer nanofibers are compatible with sub-micrometre patterning capability and electromagnetic confinement within subwavelength volumes [2,86], they can offer the benefits of organic light sources to nanoscale optics. F. D. Benedetto et al. reported on the optical properties of fully conjugated, electrospun polymer nanofibres, demonstrated the enhancement of the fibre forward emission through imprinting periodic nanostructures using room-temperature nanoimprint lithography, and investigate the angular dispersion of differently polarized emitted light [87]. Fibers with diameters intentionally produced to be in the range 0.5–5 m are used to collect reliable fluorescence micrographs and photoluminescence spectra. Confocal microscopy (Fig. 23a) on nanopatterned fibres shows uniformly bright imprinted gratings. The spectra of a typical poly[2-methoxy-5-(2 ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) fibre are displayed in Fig. 23b. in the emission of the untextured fibre the two replicas exhibit almost the same relative intensity, that is, a relative decrease of the high-energy spectral component (λ < 600 nm). Different angular dispersion for light with s- and p-polarization, namely with the electrical field vector parallel or perpendicular to the grating grooves and corresponding to transversal electric (TE) or transversal magnetic (TM) guided modes, is demonstrated in Fig. 23c, respectively. The two modes are concomitantly present in the Bragg outcoupled emission, with TM peaks blueshifted by 18–46 meV with respect to the TE peaks, indicating a lower effective refractive index (neff) for light polarized perpendicularly to the nanoimprinted features. For an MEH-PPV nanopatterned fibre, neff for TM and TE light monotonously decreases from ~1.9 to 1.2 upon increasing the wavelength from 570 to 575 nm, whereas the difference, Δneff = neff,TE –neff,TM, slightly increases from 0.01 to 0.04 (Fig. 23d).

**Figure 23.** Emission properties of nanopatterned fibres [87]. a, Confocal microscopy pictures of nanoimprinted fibres. The grating period varies from 520 nm (green GE fibre from 1:8 dimethyl formamide:chloroform solution) to 640 nm (red MEH-PPV fibre from 1:5 dimethyl formamide:chloroform solution, inset). The GE fibre was imaged under two-photon excitation at *λ* = 800 nm. b, Normalized angleresolved photoluminescence spectra of an unpatterned and nanoimprinted single MEH-PPV fibre, for various collection angles and TE polarization of the guided mode. The 0–0 and 0–1 vibronic replicas of the untextured MEH-PPV fibre, obtained by fitting this spectrum with a Gaussian superposition, are shown as blue curves. The arrows indicate the angular dependence of the emission peaks. c, Angular dispersion of the Bragg-outcoupled modes with *s*- (that is, for TE guided modes, filled circles) and *p*- (transverse magnetic (TM), open circles) polarizations. The error bar shown is the same for all points. The lines are guides for the eye. d, Wavelength dependence of the effective refractive index of the Bragg-outcoupled TE (filled circles) and TM (open diamonds) modes of the nanoimprinted fibre, and of the corresponding difference, Δ*n*eff = Δ*n*eff,TE– *n*eff,TM (open circles). The dotted line is a guide for the eye.

### **3.6. Optical sensors**

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in the range 0.5–5 m are used to collect reliable fluorescence micrographs and photoluminescence spectra. Confocal microscopy (Fig. 23a) on nanopatterned fibres shows uniformly bright imprinted gratings. The spectra of a typical poly[2-methoxy-5-(2 ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) fibre are displayed in Fig. 23b. in the emission of the untextured fibre the two replicas exhibit almost the same relative intensity, that is, a relative decrease of the high-energy spectral component (λ < 600 nm). Different angular dispersion for light with s- and p-polarization, namely with the electrical field vector parallel or perpendicular to the grating grooves and corresponding to transversal electric (TE) or transversal magnetic (TM) guided modes, is demonstrated in Fig. 23c, respectively. The two modes are concomitantly present in the Bragg outcoupled emission, with TM peaks blueshifted by 18–46 meV with respect to the TE peaks, indicating a lower effective refractive index (neff) for light polarized perpendicularly to the nanoimprinted features. For an MEH-PPV nanopatterned fibre, neff for TM and TE light monotonously decreases from ~1.9 to 1.2 upon increasing the wavelength from 570 to 575 nm, whereas the

difference, Δneff = neff,TE –neff,TM, slightly increases from 0.01 to 0.04 (Fig. 23d).

**Figure 23.** Emission properties of nanopatterned fibres [87]. a, Confocal microscopy pictures of nanoimprinted fibres. The grating period varies from 520 nm (green GE fibre from 1:8 dimethyl

difference, Δ*n*eff = Δ*n*eff,TE– *n*eff,TM (open circles). The dotted line is a guide for the eye.

formamide:chloroform solution) to 640 nm (red MEH-PPV fibre from 1:5 dimethyl formamide:chloroform solution, inset). The GE fibre was imaged under two-photon excitation at *λ* = 800 nm. b, Normalized angleresolved photoluminescence spectra of an unpatterned and nanoimprinted single MEH-PPV fibre, for various collection angles and TE polarization of the guided mode. The 0–0 and 0–1 vibronic replicas of the untextured MEH-PPV fibre, obtained by fitting this spectrum with a Gaussian superposition, are shown as blue curves. The arrows indicate the angular dependence of the emission peaks. c, Angular dispersion of the Bragg-outcoupled modes with *s*- (that is, for TE guided modes, filled circles) and *p*- (transverse magnetic (TM), open circles) polarizations. The error bar shown is the same for all points. The lines are guides for the eye. d, Wavelength dependence of the effective refractive index of the Bragg-outcoupled TE (filled circles) and TM (open diamonds) modes of the nanoimprinted fibre, and of the corresponding

Single-nanowire detection presents special advantages of high sensitivity and fast response and may offer potentials for highly localized sensing with small footprint and high spatial resolution, as have been recently demonstrated in single-polymer-nanowire electrical sensors [88,89]. And optical sensing offers potentials of high sensitivity, fast response, immunity to electromagnetic interference, and safe operation in explosive or combustive atmosphere, as well as more options for signal retrieval from optical intensity, spectrum, phase, polarization, and fluorescence lifetime [90]. L. Tong group have demonstrated that polymer single-nanowire optical sensors with extraordinary fast response and high sensitivity for humidity and gas sensing [37,91].

When blended or doped with other functional materials,polymer nanowires can be used for optical sensing with high versatilities. For instance, here a 250-nm-diameter PANI/PS nanowire was employed for gas sensing. The nanowire is drawn from a polymer-blend solutionof 2 wt % PANI doped with 10-camphorsulfonic and 5 wt% PS in chloroform and is suspended by a 250-m-width MgF2 microchannel and optically connected to fiber tapers at both ends. The sensor is operated by applying a nitrogen-diluted NO2 gas onto the nanowire witha probing light of 532-nm wavelength. When exposed to NO2, the increase of the oxidation degree of PANI resultsin spectral absorption at the wavelength of the probing light, in which the absorbance is proportional to the degree of the oxidation that increases with the concentration of NO2. Figure 24a shows a typical response of a 250-nm-diameter PANI/PS nanowire to 1 ppm NO2. A clear absorbance is observed,with a response time of about 7s, which is orders ofmagnitude faster than in other types of NO2 sensors [92]. The time-dependent absorbance of the nanowire at roomtemperature to cyclic NO2/nitrogen exposure with NO2 concentration from 0.1 to 4 ppm is given in Figure 24b, indicating good reversibility of the nanowire response. The linear dependence of the absorbance over the NO2 concentration(see inset) suggests that the PANI/PS nanowire couldfunction as a NO2 optical sensor with a detection limit below 0.1 ppm.

For optical sensing, we suspended a QD/PS nanofiber across a 245-m-wide MgF2 microchannel with two ends of the NF coupled to fiber tapers for optical launching and signal collection, as schematically illustrated in Figure 25a. For robustness, the coupling region between the QD/PS NF and fiber tapers was bonded to the substrate using lowindex UV-cured fluoropolymer (EFIRON PC-373; Luvantix Co. Ltd.), and the sensing element was sealed inside a glass chamber. To operate the sensor for humidity sensing, 532 nm excitation light was launched from the left-hand fiber taper, and measured the light output from the right-hand fiber taper, while changing the surrounding relative humidity (RH) from 7% to 81% by circulating moisture gases insidethe chamber. The optical response of the nanofiber sensor is shown in Fig. 25b, which works well with the excitation power ofabout 100 pW used here. The RH-dependent PL intensity can be attributed to the passivation of surface trap states of QDs by water molecules [93-95]. The monotonic increase of the PL output with increasing RH can be used for RH sensing, with estimated resolution (calculated from the response curve in Figure 25b) better than 1% RH. Excellent reversibility of the nanofiber sensor was obtained on alternately cycling 19% and 54% RH air, as shown in Figure 25c. The instant response of the sensor was investigated by introducing sudden changes of the humidity in the chamber, with measured response time less than 90 ms (Figure 25d), which is 1–2 orders of magnitude faster than those of RH sensors based on films ormonolayers [96–98]. In addition, the PL intensity of the CdSe/ZnS QD-doped PS matrix was also found to be sensitive to otherspecies, such as CN-ions [99]; the nanofiber sensor proposed here is promising for optical detection of many other types of samples.

**Figure 24.** PANI/PS single-nanowire NO2 sensors [37]. (a) Opticalresponse of a 250-nm-diameter PANI/PS nanowire to 1 ppm NO2 with a 532-nm-wavelength light. Inset, a close-up optical micrograph of the sensing element with a 532-nm-wavelength probing light guided along the nanowire. Scale bar, 50 μm. (b) Time dependentabsorbance of the nanowire to cyclic NO2/nitrogen exposure with NO2 concentration from 0.1 to 4 ppm. Inset, dependence of the absorbance over the NO2 concentration ranging from 0.1 to 4 ppm.

ranging from 0.1 to 4 ppm.

optical detection of many other types of samples.

estimated resolution (calculated from the response curve in Figure 25b) better than 1% RH. Excellent reversibility of the nanofiber sensor was obtained on alternately cycling 19% and 54% RH air, as shown in Figure 25c. The instant response of the sensor was investigated by introducing sudden changes of the humidity in the chamber, with measured response time less than 90 ms (Figure 25d), which is 1–2 orders of magnitude faster than those of RH sensors based on films ormonolayers [96–98]. In addition, the PL intensity of the CdSe/ZnS QD-doped PS matrix was also found to be sensitive to otherspecies, such as CN-ions [99]; the nanofiber sensor proposed here is promising for

**Figure 24.** PANI/PS single-nanowire NO2 sensors [37]. (a) Opticalresponse of a 250-nm-diameter PANI/PS nanowire to 1 ppm NO2 with a 532-nm-wavelength light. Inset, a close-up optical micrograph of the sensing element with a 532-nm-wavelength probing light guided along the nanowire. Scale bar, 50 μm. (b) Time dependentabsorbance of the nanowire to cyclic NO2/nitrogen exposure with NO2 concentration from 0.1 to 4 ppm. Inset, dependence of the absorbance over the NO2 concentration

**Figure 25.** QD/PS single-NF humidity sensors [91]. a) Schematic illustration of the sensor. b) Integrated PL intensity of the nanofiber exposed to ambient RH ranging from 7% to 81%. Inset: Optical microscopy image of the waveguided-light-excited 480-nm-diameter 245- m-long QD/PS nanofiber used in the sensor. Scale bar: 50 m. c) Response of the nanofiber sensor to alternately cycled 54% and 19% RH air. d) Typical time-dependent integrated PL intensity of the nanofiber reveals a response time of about 90 ms when RH jumps from 33% to 54%.
