bwareaopen for connverting RGB to binary

hi sir, may I know how to assign the conversion of rgb to binary. for instance,, I've used this coding but the output is all black,why is that:

      %%Binary
   im=uint(im)
   figure;imshow(im,[])
   %%remove pxls less than 100
     pixels=100;
     detectObject=uint8(bwareaopen,pixels)
    figure;imahow(detectObject,[])