Chiroptical Strain Sensors from Electrospun Cadmium Sulfide Quantum-Dot Fibers

Controllable synthesis of homochiral nano/micromaterials has been a constant challenge for fabricating various stimuli-responsive chiral sensors. To provide an avenue to this goal, we report electrospinning as a simple and economical strategy to form continuous homochiral microfibers with strain-sensitive chiroptical properties. First, electrospun homochiral microfibers from self-assembled cadmium sulfide (CdS) quantum dot magic-sized clusters (MSCs) are produced. Highly sensitive and reversible strain sensors are then fabricated by embedding these chiroptically active fibers into elastomeric films. The chiroptical response on stretching is indicated quantitatively as reversible changes in magnitude, spectral position (wavelength), and sign in circular dichroism (CD) and linear dichroism (LD) signals and qualitatively as a prominent change in the birefringence features under cross-polarizers. The observed periodic twisted helical fibrils at the surface of fibers provide insights into the origin of the fibers’ chirality. The measurable shifts in CD and LD are caused by elastic deformations of these helical fibrillar structures of the fiber. To elucidate the origin of these chiroptical properties, we used field emission-electron microscopy (FE-SEM), atomic force microscopy (AFM), synchrotron X-ray analysis, polarized optical microscopy, as well as measurements to isolate the true CD, and contributions from photoelastic modulators (PEM) and LD. Our findings thus offer a promising strategy to fabricate chiroptical strain-sensing devices with multiple measurables/observables using electric-field-assisted spinning of homochiral nano/microfibers.


Viscosity and spinnability
The carrier-free fiber formation is possible using CdS MSCs due to their lyotropic liquid crystalline polymer (LCP) behavior in moderately volatile non-polar solvents such as chloroform.
The viscosity of MSC suspensions increases nonlinearly with increasing concentration and this effect alters the flow dynamics of fiber formation during the electrospinning process. [1]Both synthesis and storage conditions of MSCs have a significant effect on the rheological properties of suspensions and the ability to electrospin fibers.The viscoelastic data presented here are from suspensions prepared from equal concentrations of freshly synthesized, and aged MSCs (sealed in a desiccator for 3 months before preparing the suspension).Both suspensions show non-Newtonian (pseudoplastic) shear thinning behavior required for fiber formation, however, we were only able to achieve carrier-free electrospinning with freshly made samples due to its higher zero-shear viscosity.

Isomers and Enantiomorphs of MSCs
The excitonic peaks at ~313 nm and ~324 nm is reported for ß-CdS and α -CdS isomers with zinc blende and wurtzite-like crystal phases, respectively. [2]Occasionally the CD transition (and absorption peak) is observed at  ~322 nm.This absorption peak is slightly blue-shifted from  ~324 nm observed for films, [3,4] but is likely due to effects of discontinuity and porosity of the fiber web.The excitonic peak at  ~313 nm and  ~324 nm is reported for ß-CdS and α -CdS isomers with zinc blende and wurtzite-like crystal phases, respectively. [2]The ß-CdS is said to be formed after the adsorption of water or other polar solvent on the surface of the cluster, due to hydrogen bonding with the oleate ligand and resulting structural reconfiguration and isomerization. [2]We confirm this transition takes place for electrospun samples due to the presence of the CD and UV-Visible absorbance peaks corresponding to ß-CdS and α-CdS isomers for fibers before and after conditioning (relative humidity 65% and temperature 21 o C) for 24 hours.The 4 scan method makes use of the antisymmetric properties to remove the contribution of linear anisotropies to the CD signal by measuring four different orientations, (

Scherrer Calculation
The q-form of the Debye-Scherrer equation is used to calculate the grain size.where K is a shape factor constant and ξ is the grain size.Couple of values of K (1, 0.939, and 0.9) was used to determine the grain sizes.The resulting grain sizes were 160 Å, 150 Å, and 144 (S3) (S4) FWHM q (Å -1 ) q (Å -1 ) Intensity a) b)

Figure S1 .
Figure S1.Photos showing the significant difference in viscosity of solutions from MSCs a) new batch (spinnable), b) aged batch (MSC dispersed in CHCl3 after 3 months from synthesis and viscosity is low and results in spraying during electrospinning) and c) Viscosity vs shear rate measurements from rotational rheometric studies.

Figure S10 .
Figure S10.Schematic showing the inversion and rotation of the fiber mat electrospun on copper sheet with a slit to take CD and LD measurements for the 4 scan method.Magnified SEM image shows the uniaxial orientation of the fibers along the x axis when θ = 0° (electrospun on drum collector rotating at 2500 rpm) Figure S11.a) Solid MSC sample, b)MSC dispersed in CHCl3, c) Copper sheet with slits wrapped around drum collector of the electrospinning setup, d) aligned fibers formed in the slits and on the copper sheet

Figure
Figure S15.a) Gaussian fitting of the peaks, b) the peak FWHM vs the q position