PKmem 650


PKmem 650 is a bright, non-phototoxic & non-toxic plasma membrane probe based on the PKmem™ dyes developed by the lab of Zhixing Chen at Peking University1). PKmem 650 labels the plasma membrane in live cells with very high specificity. The unique and unmatched feature of PKmem 650 is its extremely low phototoxicity, due to the presence of the intramolecular triplet quencher cyclooctatetraene (COT) group. It allows to perform long term live cell imaging of the plasma membrane without damaging the cells. PKmem 650 does not require any genetic manipulation, transfection or overexpression of fluorescent proteins. PKmem 650 enables multicolor imaging with SPY505, SPY555, SPY595, SPY620 or GFP. PKmem 650 can be imaged with a standard Cy5 filterset. It can be used for widefield, confocal, or SIM in living cells and tissue. Contains 1 vial of PKmem 650 (lyophilized).


Absorbance Maximum λabs (MeOH)

 650 nm

Fluorescence maximum λfl (MeOH)

671 nm

Works on fixed cells?


Probe quantity

100 stainings*

Fluorescence lifetime


STED depletion wavelength

 775 nm


 room temperature




Cells Stained with PKmito RED


PKmito RED


If you have any questions concerning this product, please contact our Technical Service department at

Q1. What is STED microscopy and how does it work?

A1. STED microscopy stands for Stimulated Emission Depletion microscopy.  It is one type of super resolution microscopy which allows the capture of images with a higher resolution than conventional light microscopy which is constrained by diffraction of light.  STED uses 2 laser pulses, one is the excitation pulse which excites the fluorophore, causing it to fluoresce.  The second pulse, referred to as the STED pulse, de-excites the fluorophore via stimulated emission in an area surrounding a central focal spot that is not de-excited and thus continues to fluoresce.  This is accomplished by focusing the STED pulse into a ring shape, a so-called donut, where the center focal spot is devoid of the STED laser pulse, conferring high resolution to the fluorescent area (Fig. 1; see Ref. 1 for more details on STED microscopy).


Figure 1. STED microscopic image of microtubules labeled with SiR-tubulin in human primary dermal fibroblasts.

Q2: Are the SPY™ probes stable at room temperature?

A2: Yes, the probes are stable at room temperature for a few days.  However, it strongly depends on the probe and the solvent.  Thus, it is recommended to store all of the probes or solutions at –20°C.



1. Liu Tianyan et al. “Multi-color live-cell STED nanoscopy of mitochondria with a gentle inner membrane stain” PNAS (2022):119 (52).
2. Yang, Zhongtian, et al. “Cyclooctatetraene-conjugated cyanine mitochondrial probes minimize phototoxicity in fluorescence and nanoscopic imaging.” Chemical science 11.32 (2020): 8506-8516.