data={{1.*10^-10,2.58022},{7.,2.66034},{7.33333,3.18999},{7.66667,3.88375},{8.,4.13342},{8.33333,4.21529},{8.66667,4.22846},{9.,4.2579},{9.33333,4.28118},{9.66667,4.28596},{10.,4.29661},{11.,4.31207}};

parameters = {[Gamma], [Epsilon], k, c, p, is};

f[x_] := (E^-k  [Gamma])/(1 + c/(1 + (x/is)^p) + E^-k) + [Epsilon]

model = Sum[( f@data[[i, 1]] - data[[i, 2]])^2, {i, 10}]

fit = NMinimize[{model, [Gamma] > 0 && c > 0 && p > 0 &&
 0 < is <= 11}, parameters] // Chop
Thread[{[Gamma], [Epsilon], k, c, p, is} = parameters /. Last@fit];
Show[Plot[f[x], {x, 0, 11.02}, PlotRange -> All, Frame -> True, Axes -> False, ImageSize -> 350], ListPlot[data, PlotStyle -> Red]]