Biotinylated Photocleavable Semiconductor Colloidal Quantum Dot Supraparticle Microlaser

Luminescent supraparticles of colloidal semiconductor nanocrystals can act as microscopic lasers and are hugely attractive for biosensing, imaging, and drug delivery. However, biointerfacing these to increase functionality while retaining their main optical properties remains an unresolved challenge. Here, we propose and demonstrate red-emitting, silica-coated CdSxSe1−x/ZnS colloidal quantum dot supraparticles functionalized with a biotinylated photocleavable ligand. The success of each step of the synthesis is confirmed by scanning electron microscopy, energy dispersive X-ray and Fourier transform infrared spectroscopy, ζ-potential, and optical pumping measurements. The capture and release functionality of the supraparticle system is proven by binding to a neutravidin functionalized glass slide and subsequently cleaving off after UV-A irradiation. The biotinylated supraparticles still function as microlasers; e.g., a 9 μm diameter supraparticle has oscillating modes around 625 nm at a threshold of 58 mJ/cm2. This work is a first step toward using supraparticle lasers as enhanced labels for bionano applications.

The modal equations for TE and TM electric field modes in a microsphere were obtained from the Maxwell equations 1,2 A sphere with 9.6 µm in diameter (i.e.average size of the SPs reported in this study) and a refractive index of N = 1.7 was used for the calculations 3 .A refractive index of N = 1 was used outside the sphere.
The WGMs are characterized by three parameters referred to as ,  and , which correspond, respectively, to the number of maxima of the radial, angular and azimuthal field distribution 2 .First, the modal equations for the electric field (TE and TM) were solved for a wavelength close to the observed lasing peaks (λ = 633 nm) in order to determine the  numbers for which the resonances occur (interception of the modal function with 0).
Once the values of  are known, the exact resonance wavelengths of the SP (within the lasing region) can be found.Recurring to the modal equations again, this time using the highest  number within range ( = 1;  = 74 for TE and  = 1;  = 7 for TM), we obtain  = 631  for TE and  = 633  for TM (interception of the modal function with 0).Note that other resonant wavelengths within the lasing region are also possible (e.g.{ = 2;  = 65}, { = 3;  = 61}, … for TE modes, and { = 2;  = 64}, { = 3;  = 60}, … for TM modes), but for the sake of simplicity we will focus on  = 1.
Likewise, for illustration purposes the azimuthal field distribution, , was fixed at  = 50 for the TE and TM modes.
Once the parameters above are set, the electromagnetic field can be evaluated and plotted.Figures S1 and S2 show the real components of the Electric field (a.u.) for the modeled sphere.
The effect of the Si shell on the SP surface was neglected due to its thickness (thin layer) and refractive index (N = 1.55 -1.40), which sits in between CdSSe and air.

Calculation of lasing thresholds as a proportion of beam spot size.
The beam spot size was measured using a thorlabs CCD beam profiler (BC106N-VIS/M).The beam was elliptical with a beam radius of 39.9 µm and 22.5 µm measured at 1/e 2 of the intensity.To calculate the beam intensity incident on a SP, the multivariate distribution of the beam was calculated, as can be seen in Figure S10.The double integral was then taken for this function over the size of the SP, assuming the SP lies in the center of the guassian peak.The result show that only 16.7% of the beam intensity would interact with a SP 9.3 µm in diameter.

Figure S2 : 4 . 2 Figure S3 :
Figure S2: Size distribution of SP, SP/SiO2-NH2 and SP/SiO2-PC-biotin.Size distribution obtained from the secondary electron images obtained in scanning electron microscope.The size distribution of the SPs was fitted with a lognormal curve.Error bar calculations for Figure 4.The experiment was repeated 3 times, with 4 x 5 µL of SPs dropcast onto the functionalised substrate.The 4 individual 5 µL spots were highlighted on imageJ software and the mean pixel intensity function was used.The mean value was taken from all 3 repeats of each of the 4 x 5 µL spots.The error bars are the standard deviation of those values.Error bar calculations for Figure 5, Figures S6, S7, S8 and S10.The error in fluence considers both the error in measured beam spot size and measured pump energies.The error in fluence was then calculated using the propoagation of error:

Figure S6 :
Figure S6: Extracting the l-numbers from the modal equations and finding the respective resonant wavelengths for TE modes (example given for l=74,  = 631 ).The real component of the Electric field (φ direction, in spherical coordinates) was then calculated at the xy cross section and xz cross section of the sphere (blue circle) for  = 50.

Figure S7 :
Figure S7: Extracting the l-numbers from the modal equations and finding the respective resonant wavelengths for TM modes (example given for l=73,  = 633 ).The real component of the Electric field (θ direction, in spherical coordinates) was then calculated at the xy cross section and xz cross section of the sphere (blue circle) for  = 50.

Figure S8 :
Figure S8: Laser transfer function plot of SP for mode M2.The PL intensity was integrated from 634 nm to 636 nm.

Figure S11 :
Figure S11: Distribution of peak lasing wavelength for each sample: SP, SP/SiO2 -NH2 and SP/SiO2-PC-biotin.The maxium and minimum wavelengths are shown by the extremities of the plot, the green boxes represent the standard deviation, and the mean is represented by the central line in each green box.

Figure S12 :
Figure S12: Distribution of laser threshold for five separate SPs for each functionalization step.The maxium and minimum thresholds are shown by the extremities of the plot, the green boxes represent the standard deviation, and the mean is represented by the central line in each green box.

Figure S13 :
Figure S13: Multivariate normal distribution of incident beam.

Figure S14 :
Figure S14: a, Spectra of neutravidin bound SP/SiO2-PC-biotin.b, Laser transfer function plot of neutravidin bound SP/SiO2-PC-biotin, the PL intensity was integrated from 629 nm to 635 nm.